I bought Kingdom Hearts after seeing the trailer, and it turned out to be a decent buy. I also bought Deus Ex: Human Revolution on a whim, with no reviews or nothing. That, too, turned up to be a good deal.
Other such games include Tomba 2, Vagrant Story and Alpha Protocol.
Bad buys... Well, those are few and far between, but there are a couple of titles I grabbed on a moment's notice. The most recent one would be Operation: Raccoon City. The PlayStation Store had it at a 4.85 or something, and even though I know not to trust these numbers I still went on ahead and bought the damn thing. Worst money I've spent in a while, at least for now. I still might get into it later, and find out it's a decent title after the awful first mission.