1st off this is coming from a melee sheik/zelda main, and a current sheik/zelda secondary.
Sheik is, well, a LOT worse than melee. However, she isn't bad. As you guys said, upsmash is her most reliable kill move. Her priority is ok, I have seen better. Thankfully she's a combo master to compensate for the lack of killing. *cough*ftilt lock*cough*. She reminds me of fox in brawl, except she can't kill well. They both can combo great, Both have a decent recovery, and both are light(bad thing, early deaths lol.)
I played Zelda and sheik together in melee as well(thunder kicks for the win). What keeps sheik viable for higher level play the invaluable ability to transform to Zelda, who can't combo well, but can kill like no tomorrow. I agree about the SBR discussion, about how Sheik and Zelda are great together. Its like pieces to a puzzle, on one hand, sheik, the fast, crazy comboer. On the other, Zelda, slower, but also known as the princess of KO's. Of course switching isn't the most practical thing in brawl. Its long, and your vulnerable, so you can only use it at ideal times.
Imagine for a second if the transform time was as fast as it was in melee.....
Now you not talking about well rounded character pair, but a GREAT one.