I'd say Borderlands 2, but I got it free as part of my video card so it wasn't really a purchase. Didn't enjoy it much, though.
Other than that I've been pretty happy with the games I bought. Got a few duds in bundle deals, but the deals were so cheap I wasn't expecting much anyway and I got what I paid for. Even the worst of them were entertaining long enough to be worth the cheapo prices I got them at. I haven't really bought any big budget games lately, though.
Starfarer.
I like the arcade action of the battles, but that's all that the game has going for it. It never quite managed to go as big as it was envisioned, into multiple worlds and dynamic economies, etc. Additionally, it seems like development is headed down the boring RPG path into making current gameplay features harder to unlock through a grindy character-leveling system, rather than actually adding more gameplay.
I have probably narrowly played out my $20 worth of the game, but it hardly compares to the windfall returns of gameplay I grabbed for $12 when I bought Mount and Blade at version 0.632
Starfarer is only on version .54, the campaign for it is a placeholder single system with a few stations and no meaningful game play outside the battles has been added yet. How can you write that off as a regret when it's so early in development? You may as well say "Well, that kid was supposed to be smart, but he never managed to finish high school" when the kid is 3. M&B was pretty crap early in development, I remember versions with nothing but a town called Zendar and fighting little groups around it and no real goals.