Sheik's completely glass cannon, so fragile, but in the right hands (TKD would be a great candidate, since he's used to playing a character with as light a weight + high susceptability to CGs/locks/juggles) can do some work.
imo Sheik thrives when she from the ability to outperform the other character in damage, I think she shares a similarity to Diddy in the sense that both have a really good overall moveset that's slightly offset/upset by lack of a strong/reliable killer that finishes below 140% on most opponents, although Diddy seems to obviously polish the opponent off better because of bananas. When you see a good sheik play though, it's apparent that killing isn't the biggest deal with sheik when the opponent is suffering from an initial 60% difference in percent. She also has fairly reliable setups to her later, but more reliable kill moves like dsmash and nair (via jab cancels and the like). Things like ftilt staling also can be really nasty, as staled ftilt -> usmash will kill really early, in fact (sweetspot usmash is one of the strongest vertical killers in Brawl)
That and sheik's edgeguard game is pretty nice, not MK ability, but pretty nifty against some higher tiers with average recoveries (Izaw v. Haze is a good example of how even Marth has to worry about a sheik offstage, although not entirely reliable to determine the entire MU based off of that).
I also think vanish as a kill move is a tad underrated. reading with this attack is sooooo basic, the trick is that when done oos, it looks almost exactly like a spotdodge, and most opponents try to punish it that way. Difference is, you want to punish vanish at the very beginning when it doesn't have invincible frames, as opposed to the end.
She also has high "anti-camp" ability with needles, not unlike Snake's grenades. They don't completely forgo approaching and reverse the status quo, but they can still do some damage to an opponent who thinks he can hard camp all the time. 18% is nothing to scoff at when getting nailed by these, which are also transcendent.
Problem is, she has lots of decent MUs, but then gets lol'd by some really weird MUs where her game all of a sudden becomes unsafe/unrewarding, like Kirby or Pikachu. Her offstage game is pretty bad, not unworkable, but definitely flawed, which doesn't help considering her fall speed and light weight make for a bad combination.