Wow...a GP8...I had one back in the late 80's. In the early 90's I had an SE-70. By mid 90's I'd gone to pedals and tube amps, which I am still using....but I recently bought a GT-100. I used it for about a month, and ended up selling it on Ebay, although it wasn't a bad unit.
Programing is very easy...probably the easiest I've seen on a multi-unit....two fairly large screens, with 8 knobs assigned to each parameter shown in the screen, kind of like being able to see 2 FX pedals at once with 4 real knobs each.
FX such as pitch shift, delay, modulation, etc..were very nice. High gain modeling was very nice too. But what I found was that I did not like it at all using several of my tube amps...a hand wired deluxe reverb clone, and a blackface dual showman clone. I found the cleans were OK, but mid-gain sounds were lacking. Not the case when going through full range monitors or through headphones...that sounded good. Just didn't like it through a guitar amp (even with the amp compensated EQ settings on the outputs).
What was really neat is that the GT-100 is also a USB audio device, so you can plug it into your computer and use it with a DAW. It will allow you to monitor what you're playing without having to go through the DAW, so no latency, and the outputs are stereo outs from the DAW for monitoring other tracks. Very cool in that respect.
It's no axe fx, but at $550, you can't really expect it to be.