I have recently finished putting together my first cab, and I'm using a 27 inch Apex TV that I really enjoy. Almost all games look pretty good on it, although trying to actually compute (as in do anything else on the computer) is very difficult when using a TV as a monitor. I think the bigger screen adds quite a bit to my favorite games, like Gauntlet, Tempest, and Street Fighter 2.
If, as paigeoliver suggested, some games are too big, I believe you can run games using a smaller display size by fiddling with the settings in Mame. I haven't done this, though, and my scores in Galaga are so awful to begin with that I don't notice if they're better or worse on a big screen. I think it's all what you get used to. Personally, I've run games from the early 80s to the most recent stuff, and I think it's all pretty fun when played at 27-inch size.
BTW, the Apex TV I got is super cheap, at only $130. A lot cheaper than either a big computer monitor or an arcade monitor, and it looks pretty good to me.