I had one, as well as pretty much all of the other mid-tier effects units; Line 6, Digitech, Zoom, etc...
The GT-100 has *the* fastest patch changing in this category. At least it seems to from my experience - no dropouts when changing the presets, no lag, seems instantaneous. This is important if you plan to use it in 4 cable mode, because you don't want your preamp being muted every time you change a preset on the downbeat of a chord or note.
The tap tempo behaviour is frustrating. The delay time doesn't smoothly transition like with the Zoom units, the TC Flashback X4, the Nova Delay, the DL-4, or (confusingly) the Boss DD-7. So you're going to get very strange pitch artifacts like an old bucket brigade delay pedal. This was kind of a pain in the ass for me, which is why I sent mine back.
Having two screens makes programming the unit extremely easy. It's the most user friendly unit on the market in this respect, being out several of the higher end units.
You get about 12 slots iirc, more than enough if you just want to use effects. Most of the effects are servicable; not earth shattering like you'd get from Strymon or someone like that, but good enough.
Most importantly, the tonal colouration is very transparent. More transparent than the Line 6 HD500.