Siesta Key remains one of the nicest beaches. yyz is probably referring to the red tide that happens every once in a while. you never want to go if you know the red tide is there. there is dead fish on the beach each morning, the smell is horrible and noone goes to the beach. its basically tiny organisms that are deadly to all the sea life. don't know the cause or the frequency but i was down there last summer and it was there for a little while.
otherwise, the sand is great, the scenery on the beach is pretty good as a lot of college chicks gather around. the nightlife isnt all that great, as mentioned, there is a village type area with 4 or 5 bars in a row.
if you are looking for a quiet, good time though with the fam or whatever, this is a good place for that.
just try and find out if the redtide is going to be there when you want to go.