I love co-op, as it has made REALLY mediocre games (Fallout: Brotherhood of Steel, Baldur's Gate: Dark Alliance) fun to play. In the last year, I've seen a slight increase in games that has built-in co-op.. The only problem is that none of those games were the top titles (like Doom 3 or Far Cry) so it might not have as much of an impact as one would like.
There are definitely not enough RPG's using co-op. Sacred is a shining exception, but that co-op is so buggy it's almost impossible to play anyhow.