The one major drawback for Siesta Key is parking. Finding a place to park can be excruciating, especially on weekends. What I suggest is to park somewhere away from the main beach and business areas and bike from there. Biking around the Key is quite a pleasurable experience since the area is very bike friendly. And as a note, there is a pay-to-park lot just south of the main beach parking with a beach access trail, but it is at least 1/2 mile trek to get to the beach on that path. But surprisingly enough, even with all the traffic and parking woes, the beach stays fairly uncrowded as it is quite large.
One of the major pluses for me is the volleyball community there. I have played in several at Siesta Key and I love jumping out of the sand as it is more shallow and packed than our beaches. It lets me hit like I am in my 20s again!
Overall this beach gets a 9 out of 10 for me and it is probably a top 5 beach in Florida if not the U.S. So if you are ever in that part of Florida I highly advise stopping by.