I do it often, but I shouldn't
Just got Deathspank for PSN and it is hilarious. Great impulse buy. Same goes with PSplus (I like it, and I refuse to respond to any hate about it that is not backed by a good argument), never would have got WipeOut, and I've already sunk 25+ hours into fieldrunners. I like my tower defense.
Usually, I get awesome stuff. Except for PowerDrome...but S.L.A.I balanced that out with huge mounds of epic awesome!
I usually plan my BIG game purchases (stuff like inFamous or Starcraft II(just listed these games that got hype off of my head, don't blame me if I seem biased or something)) ahead of time, but smaller titles find their way into my shopping cart (digitally or real). They must have legs or something...