A friend of mine got a GT-6 and Yamaha DG stomp and was asking me which one he should keep. (He kept the DG stomp and I thought he was dead wrong) He wanted the overprocessed direct sound, and the DG was better at that I guess. He already has a big pedalboard for "pedals to the amp" tone. But that's what I liked about the GT-6. It sounds like pedals to me, not a processor. I think you can hit the front end from the guitar, so you have the overdrive and compressor "pedals" before your preamp. Then loop the preamp so it's placed before the effects. If I didn't have so much gear already I'd love one of those to use with a good tube amp. But you have to like boss sounds. I love the super overdrive, and it gets you that and the DS-1 if you like that. It doesn't do that good of a job with the tube amp modeling. Those sounds are pedal-ish too. I mean they are pretty good sounds, but not like the "total amp model" that you get with some of the other boxes. I like that. I think it's better that way. It's a more pure sound to hit your preamp with. So you might find yourself using a plexi model instead of a super overdrive to goose up the preamp. There's lots of silly effects you may never use, and I wouldn't trust their wah modeling, but it might be a good pedals alternative for you.
I've got the 21xx as my effects brain, and to me, so long as you loop everything so it can be out of the signal path when not in use, I find it's chorus, delay, and reverb to be totally acceptable. So I keep it as my rack brain because I like the Conrol One also. I remember the GT-6 having a good "bypass" sound, so you might not need to loop it to avoid signal degredation. I remember A/B-ing it with plugging directly into the amp and it was hardly different.