I record with the X3 all the time. It sounds great for electric guitar and bass -- like any technologically based piece of gear, you do have to know what you're doing with it. I haven't run it as a keyboard amp or anything like that, but I'd assume it's pretty good. I also haven't done much with the vocal stuff. No real need ... I've got good pre-amps, dynamics processors and FX.
As for using it instead of an interface, it wouldn't be my choice if I had a good interface. People on the Line6 forums say its USB audio can be kind of unreliable. But if it's all you've got, I imagine it would work.
You know, if you own the X3, you automatically qualify for free download of Pod Farm Platinum, which is their $249 plug-in. I owned the X3 for a year before I realized that. Pod Farm gives you everything the X3 gives you, but as a plug-in. So if you get a perfectly performed take on guitar, but you you come back the next day and don't like the way it sounds, you can just re-amp it instead of re-tracking it. You could do the same thing to check out the vocal processors ... just record something direct to computer and then try different pre-amps, FX, etc. after the fact to see if there's anything you like.
As for whether it's a good-enough sounding piece of gear ... Lincoln Brewster, arguably the best Christian guitar player in the world (and the first lead guitarist for Steve Perry's post-Journey projects), not only uses the X3 exclusively in the studio, he also plays it live. Since Mr. Brewster can afford to play anything he wants, it says something about the quality of tone that's possible with the X3. So take that for what it's worth.
