(Again, my lack of creativity startles even me :D)
Sheik's style takes a bit of a turn from Fox's in that he is a bit more versatile. He can do excellent with many tilt attacks on the ground and his smashes can lead to other attacks, his throws can be abused in combos on the ground as well. However, Sheik shines most in the air, where he can sometimes manage to COMBO. Not juggle, COMBO. :D Sheik has a few more options than Fox does due to the variability in style. However, Sheik is DEFINITELY not a heavy specialist; due to the following reasons.
Up+B: His only recovery move. It is a bit of edge hogging bait because it usually doesn't get you past the edge easily.
F+B: His chain, although some may prefer it, doesn't do a whole lot of damage or knockback, and has a lot of lag AND recovery time.
D+B: Transforms into Zelda, which can be handy for a couple of reasons later mentioned.
Neutral B: Sheik's needles are handy in cancelling out projectiles, and they can be excellent edge guarding tools, especially against the Fox team, CF, or anyone that has a recovery move which has enough lag to predict when they will reach the level of the field. These are primarily used to disrupt the opponent's rhythm, and if you quickly counter a projectile with a full set of needles, you can run into them and combo at will. It is Sheik's most useful B move.
Recovery: Sheik's recovery is nothing to brag about whatsoever, but remember Sheik's trump card: If sent VERY far off stage, instantly transfigure into Zelda. His Up+B has the best distance for being a single move, and has directional variability which can be helpful in a variety of situations. Also remember this: By the time Sheik is so far off stage that she requires this plan to be used, you must transform into her IMMEDIATELY and use her Up+B when you get near the stage. Zelda's floatiness attributes to the increase of the technique's usefullness. Try to get familiar with Zelda, because it isn't likely that you will have a chance to return to Sheik until you KO them.
Counters: There is no character that is statistically advantageous over Sheik, AND unlike fox Sheik has no weaknesses that any particular character can take advantage of. This is why many people regard Sheik as being better than Fox. Also Sheik has no counter-stages. He can wall-jump, so he can easily save himself from falls in places with columns of walls, and can easily manipulate himself around areas because of this. Fox can walljump, but this is offset by his falling speed which makes this only useful for recovery.
Juggling: One of Sheik's many strong points is that she can make a juggle look like a combo, and vice versa. Few attacks can out-prioritize his aerial attacks (save the aerial down+A), and two of them have a very useful sweet spot location. Also if the sweet spot doesn't hit, the knockback is significant enough that he can fast fall and attempt another attack. The most notable thing about Sheik is that almost ANY attack used can lead to almost ANY other attack; which is why everyone complains about Sheik's unpredicatability.
Neutral A: Basically, take the same strategy as Fox does. Don't let Sheik engage his rapid punch, but use the neutral A to small-string or use it once/twice to lead into a tilt attack then a smash/throw, or skip the tilt attack altogether. Also lighter characters tend to not be affected by the neutral A's usefullness, just as Fox's. And lighter characters usually dont get tilts used as much on them, but after a couple of tilts Sheik can already begin to juggle incredibly close to the ground.
Dash+A: This attack can be countered if noticed, so try to make the usage of it as unpredictable as possible. It has somewhat of recovery time, so if they happen to sidestep, get ready to get pumelled with just about anything. This is Sheik's second most damaging weaknesses, but remember: it isn't a necessary move.
Forward A: This attack takes part in the majority of Sheik's combo's. It has decent stun, great recovery, long execution time, no lag, and can easily lead into his tilt up or a throw/smash/air attack.
Down A: The least useful of his tilts, but it is by no means a bad move. It's range can cancel out a surprising amount of attacks, and against lighter characters gives Sheik the opportunity to do a tilt up+A or start his amazing juggling schemes.
Up A: This move, coupled with Forward tilt A, can add much variety to his many combos. In some cases, you can choose to either use this or use forward tilt A, but in most cases you must choose one over the other. At any rate, it is very useful, and can quickly lead his to an aerial back+A.
Smashes: Sheik seems to receive a lot of ridicule for the lack of strength in his smashes; and this is true. But his superb aerial abilities makeup for this weakness, and it isn't something that a character can really exploit anyway. However, people just seem to have the misconception that Sheik's smashes were meant to KO; they weren't. They are to end/begin/continue combos that eventually get the opponent into the air for his air techniques do to their magic. And if you connect in a sweet spot (which can sometimes be done surprisingly during a combo), then you will notice just how effective his up and down smashes can be.
Up+A: This can be used on an opponent above and to the right or left in addition to directly above him. It isn't extremely common for a Sheik to use this more than 7 times in a 20 stock match, but it is not a move to discredit.
Down+A: This attack's main purpose is to ward off someone who is putting the pressure on you, seeing as it can attack some people to the above+left/right as well as people in front/behind. It is a clearance move, much like Fox's except less power, and much like Jiggly's except more range (and some power deduction). Sometimes it can begin combos, depending on the distance the opponent travels.
Forward+A: His least useful smash attack. If used properly, it can continue, begin, or end a combo. However, if used incorrectly, it can lead to dire consequences; such as people having the time to attack from behind since it lasts so long and doesnt attack from behind. This attack doesn't have much lag or recovery, but the fact that it is a two-hit attack can leave Sheik at a disadvantage if this is used wrong. This is probably Sheik's biggest weak point; use sparingly.
As you might infer, his smashes are differently used than most smashes are. But they blend into his game perfectly.
Aerial: This is where Sheik shines (as if he didn't shine anywhere else...)
Neutral A: Most likely his second least used aerial move, second to his Down+A. This is because his Forward+A can hit much better, and the situations where this is necessary is very uncommon.
Forward+A: An attack that can out prioritize many attacks and has amazing knockback. It is swift and probably Sheik's strongest move that doesn't require a sweet spot to get excellent results.
Down+A: An overall bad move; bad lag and recovery time, sends the opponent in an awkwardly up position. Easily out-prioritized.
Up+A: An attack that can KO is so many situations. It has a sweet spot that, if used, can KO under 100%, and has good execution time, no lag and little recovery time. Its regular knockback is also remarkable.
Back+A: This attack is usually used when the opponent is closer to the horizontal doom than vertical, and for good reason: It's decent sweet spot range has great knockback, and it can be used to fast fall and go into another attack. However, if you are facing your opponent, try using his aerial Forward+A instead.
Throws: Sheik's throws are some of the most combo-ready throws I've ever seen; and Sheik can use them in multiple situations.
Up: Great for setting up his deadly air game stratagems.
Down: Decent throw comboer, but Fox's beats Sheik's to the punch. Not bad, though.
Forward: Pretty good combo-ready throw; good to use when following up with a Dash+A.
Backward: Same as Forward.
Overall: Sheik's variability and strengths, followed by the fact that she has the fewest exploitable weaknesses, and NO counter characters+stages, is why people tend to believe that Sheik is better than Fox. The lack of weaknesses is a valued strength in itself.
Fox and Sheik are both very effective characters, but Fox's gameplay is more specific than Sheik's, thus Sheik is harder to counter. Also, Fox has more weaknesses than Sheik does, and there is no way to take advantage of a practiced Sheik player WHATSOEVER. This is why Sheik is regarded as better than Fox and this document of mine was created to prove that.