I've been reading a lot of Urban Fantasy lately, and most of them are in first person, and I've been really liking it. I think a lot of it depends on what kind of story. Is it a smaller story about 1 person specifically, or is it a big story that really needs a wide scope to tell it? If it's the former then I think it first person is OK, but for the former I think third person works better. Which probably why you don't find a lot of first person stories in Trek, they tend to be bigger in scope than just one person's experiences.
As for the multiple first person narrators, I didn't realize how tough that would be. I'm working on a first person story right now, and I briefly considered adding a second narrator, but then I ran into the problem mentioned up thread. I just wasn't sure if I was comfortable enough to try to come up with a second narrative voice, especially since I'm still trying to solidify with the first one. There's also the fact that pretty much the whole story revolves around the first character anyways, so there really wasn't anything worthwhile that a second narrator would add, other than the brief novelty of getting into that character's head.