Meh. Like most things, I would say "shit's situational".
Usually (especially noticeable in story heavy games - see any decent RPG), good voice acting can add tremendously to the style and quality of the game. Wether you think it was good or not, you can't really deny that Final Fantasy XII's storyline would have had the same feel without the whole European accent they went out of their way to give most of their characters. They were going for a specific feel and voice acting went a long way to make that not only more pronounced but more believable.
On the other hand you have, as previously mentioned, things like Mario games in which you really don't need voice acting, nor would it add a whole lot to the game. Anything more than the occasional sound would feel a bit out of place.
While it's entirely true that bad or just plain odd voice acting can completly ruin your enjoyment of a game(dear god I wish I could turn half the voices in Kingdom Hearts 2 off), I don't really think it's something significant enough to worry about. Voice-acting is an extremely recent addition to games and we're already seeing the average VA quality rise considerably and quickly.
I think it's very similar to the way the whole move from 2D to 3D was. As it gets more and more common place to actually have some more consistent standards of how voices should be in games and what should be done to make sure the voices sound like they're supposed to sound, the quality of VAs will just get consistently better, until it gets to the point where, with the rare exclusion of those extremely bad games that will always exist, the voices being bad won't really much of a concern anymore.