what type of unit do you want? big complex, small? fancy pool? quiet? etc.
You mentioned SanDestin. It's nice, but very pricey also. But nothing wrong with it besides being higher than average on price.
My personal favorite is Crystal Sands. It's small, quiet and simple, but nice at the same time. But there are lots of other nice units also.
Abbott Resorts (a.k.a Resort Quest Northwest Florida) has the corner on condo rentals in Destin. I've rented from them multiple times and always had good luck with them. I've got some $50 or $100 buck off coupons for them somewhere if you end up renting from them. (I could mail it to you or something).
When in May were you going down? (we'll be down there diving in May).
mike