You're going to get a myriad of opinions from a myriad of users with multiple experience and application here.
With that said, using it to do small gigs where I need simplicity and versatility I love my GT-6. You can always find them cheap and it is simple to use - I have yet to read the manual because everything was intuitive enough.
With that said I have a Pavlov drool for the GT-10. The 6 has just whet my appetite and I can see where the 10 has more flexibility, USB, etc.
I tried the Zoom unit,,,can't remember that model,,,what the 9.2tt,,,something like that and also TC's Nova. Liked them both in the showroom but I guess not enough to replace my GT-6, so for now I'll wait until I can afford the 10.
With the Digitech, Zoom, BOSS, TC, Line 6, VOX and others I think they all make great and flexible boxes - the technology and palette of sounds is there. I have come to learn with these units that if I'm not happy with the sounds I'm getting it is often my failure because I haven't learned the device or set some parameter right. The most important thing is you'll have to spend some time store testing and and much time learning the unit and making it work for you.
Practice + experience = tone