I know lots of people on here have complained about them, but my experience has been great. I use mine for exactly what you're describing, to switch channels/boosts etc. on my amp and switch effects patches at the same time to eliminate tap-dancing (I sing lead in my main band and usually at least backup in the others as well as playing lead so that can be a real pain in the ass getting in and out of things with pedals). When I was using my strat primarily, I ran an OD808 in one of the loops up front and then the other FX in the fx loop. Though there is some tone loss (any time I use a loop at all I hear some honestly, but the xtc classic loop is very good).
These days I've been leaving the strat at home mostly and using primarily my 335 though will be trying to work the charvels in there too as they're a lot more stable tuning-wise and when I'm singing lead I don't want to stop and tune at all if I can avoid it. But as a result I haven't been using the od808 at all and so I'm just running the g-system in the loop. There is something I prefer about plugging straight into my amp and then having the FX loop but that negates using the loops for pedals out front (which isn't an issue as I'm not using any, but if I wanted to, would use the loops).
A lot of it was just setting up the input/output levels by A/B'ing with the g system out of the signal chain entirely, and then comparing it to just the amp with a 1/4" cable in the loop and nothing else. In my case that small little thing I don't like is there with just the loop so the effect the g-system has is negligible and at gig levels in the mix, not noticeable for me. Balanced cables for some of the conncections per laird's white paper made a huge difference too. My setup is *quiet*.
A good friend of mine who is successful in his own right and tours with several heavy bands bought his g system after seeing me with mine at gigs etc. and I helped him get his going and he loves his too. So yeah, it does exactly what I want and I think the FX themselves sound phenomenal but definitely YMMV. It's not a quick plug n play solution but it does a great job of eliminating tap dancing and letting me switch to any combination of channels/boosts/fx I want with one stomp. I also like that the floorboard is powered with the cat5 cable so setup is quick and neat on stage.