They're okay cards for the price. Where they really shine is performance compared to power draw, but other than that it's a pretty average card.
Depends on your uses though. If you want to game, and that's all you can afford, I'd probably advice you to get a new card. If you're a light gamer, play mostly online games like WoW, or need a GPU for some other not-as-intensive purpose, the 7770 is probably good for your needs.