I love local MP in all forms - grouping up or taking on a friend is the most fun you can have while gaming.
Online multiplayer is a double edged sword - it DOES elongate a game's lifespan by a mile, however you do have to face issues like lag, hackers and douchebags on a daily basis (for most games, anyway, especially FPSes). I also hate, hate, HATE that online multiplayer is replacing local MP (sells more consoles and games...)
MMORPGs are also a bit of a double edged sword - while the social aspect of something like WoW can make a game feel like playing with friends (but not quite), the games tend to suffer losses to the 'RP' aspects, trying to make a game that will appeal to everyone.
Multiplayer is how gaming should be played, but I'm not sure we're going entirely in the right direction. The Internet is great and all (hi Escapist

) but as we all know -