I use one regularly.
The harmony doesn't work if you simply play bass into it, it needs at least the third or minor third of the chord as well to work out what harmony it should be doing. I tried tapping a feed from one of my guitarists 'tuner outs' which worked to a degree but relied upon my vocal phrasing being exactly timed with his playing...easy to do if you are both singing and playing guitar, but not as easy as you think to play bass, sing and time your vocals exactly to what the guitarist is doing.
However, you can set the key of the song manually, which is what I do for certain songs and there are other voices like the octave up and octave down that don't need any pitch information since they just track whatever you're singing. I use it because I'm the only one in the band that provides backing vox and there are some songs that just need some vocal doubling or richer harmonies than a single voice. It worked well for that but it was a bit of a learning curve.
It did convince me that the technology was worth sticking with and I recently bought a TC Helicon VoiceLive Play, which is the next generation device. For what I need the VoicePlay live is really impressive. Much more intuitive, more flexible and can take it's pitch info from built in mics or from an mp3. For bands like ours where we don't have a sound engineer, have to mix from the stage and have to kick in any effects with our feet while playing the VoiceLive is really good at what it does....
Trouble is I'm still using the Harmony GXT because I've loaned out the VoiceLive to our lead vocalist until he gets his own.
So the GXT is OK and will do what you want with a bit of setup and patience, if it's a good deal it will give you a good taste of whether these devices are right for you.
