I have a Vox Tonelab SE (as opposed to the smaller desktop one). Before I bought it I had tried pretty much every other modeller out there at the time. And I don't mean just in the store; I took most of 'em home. Firstly, the Digitech ones suck. The sounds are just awful. The POD stuff is good, but I can clearly hear that the sound is modelled, rather than from a real amp. I've sat through blind tests on the net where people have A/B-ed real amps with the POD XT and an amp of the same kind as the one being modelled and I can always tell before the clip is even half over which is which. Hell, even my wife can and she's a keyboard player, not a guitarist. The POD does have a good distorted sound, but I can still hear the difference between it and a real amp.
The GT-8 I didn't get a lot of time to try out. It seemed to be pretty good. Much better for effects than possibly any of the others, but I still didn't really like the amp modelling very much, though it was miles in front of the Digitech GNX4.
I couldn't get a Tonelab SE in town to try out, so I ordered it based on clips I'd heard online and reveiws and such. But I'm so glad that I did! It has the closest to genuine miked amp sound that I've ever heard. It even sounds better than a real solid-state modelling amp (like the Line 6 Flextone) to my ears. Although that does depend, of course, what you plug it into. I find that when I play live and plug right into the house system, the Tonelab SE sounds excellent. It feels very much like a real amp and I'm extremely fussy when it comes to my sound. It's also more portable than the real thing, though maybe not by too much--these things are the biggest, heaviest multi-effects-type units around!
Speaking of effects: they're adequate. I am not over the moon about the effects, but they get me by. I tend to use only a few effects anyways. But that's not to say that they're bad! I do like them, but I would prefer a few boutique stomp boxes--who wouldn't, really?
Still, it's not as good as a real amp and a couple of 4x12s, but then again, I can't really afford a Hughes and Kettner Triamp stack right now and I don't think I'd get to use it to its full potential at the places I play these days. Plus it would be even less portable.