I have a UX1 and it's a cracking piece of gear. You don't need a mic, no, unless you're recording vocals or acoustic guitar. You just plug your electric guitar into the 'instrument' input and go from there. It's USB powered and the Gearbox software has a great selection of amp models and effects (make sure you download version 3 from the Line 6 site though). It can also act as a very low latency soundcard for if you want to run anything else through it, like Guitar Rig or Amplitube. I use Guitar Rig 2 and where I had loads of latency issues before, I now have none using the Toneport as my soundcard in the GR settings. I also sometimes mix and match Line 6 amps with GR cabs. Gives me more versatility :D.
The only downside I can think of is that you can't buy a footswitch for it. I just play through my PC speakers for the moment, and the sound is great, but you can't buy a footswitch that'll change patches if you're jamming to a track you've loaded into Gearbox. But it's a recording interface so I guess it won't happen in the near future.
Anyway, just my thoughts :).