I have one, and I have had a good experience with it. But in my latest setup I went for a Ground Control + GCX. Partly out of curiosity, partly because I am using more individual effects, rather than an all-in-one. Here are some comments I sent another board member:
The Gsys is nice for what it does, but it does have limitations. If money is a concern, you can get a G sys used for like $700. The iB mod looks cool, but most people won't be able to tell the difference. My opinion is that there are a couple issues with the standard G system front-end that people have complained about, so it probably addresses them to some degree. But I also see that the price point of the Gsystem is dropping over time, and TC needs to add feature to keep the price up. I think you would have a hard time telling the difference between the std, LE and iB versions. Having written that, I personally would probably buy the latest version just to get all the possible features.
I do have the brain in my rack. It is really nice to have all the electronics in a rack and just the G controller plus an expression pedal or 2 on the floor. It's small and powerful, and only has one cable back to the rack, which is really convenient.
HOWEVER, the G has some limitations.
1. There are 5 switchable loops and you have to put your amp in one of them, so you can only switch 4 loops. There are only buttons assigned on the controller for 3 loops. You can assign another button to switch the 4th loop, but you have to give up a function for that. I recommend the compressor, which I don't like. If you have 4 or fewer pedals, you're set. If you have more, you're going to have to add a GCX or something. I wish there were 3 more loops and 5 more buttons.
2. There are 4 relays for amp switching. You can set the relays into only TWO patterns for any one patch. This is a bit complicated. The bottom line is that you can't switch your amp functions independently in a given patch. You have to switch patches to change amp settings. You do get a second setting, so you could, for example, toggle between two amp channels, or SOLO on and off, or FX LOOP on and off inside of one patch, but that is all. And you would have to assign a button from one of the other functions to control the relay toggle. Many people use an amp gizmo instead.
3. The effects are in the order they are in, and you can only use one effect per block. So you cannot have phase AND flange at the same time, and you cannot put a modulation or delay effect in front of the preamp. If you want phaser or flanger pre-distortion (like Van Halen), you have to use a pedal or anther rack FX box.
4. The preamp section is MONO. You cannot use two preamps in stereo. Only the FX loop is stereo, and even that is only MONO in, but stereo out.
Read the G system forum, and especially Laird Williams's white paper for setting up the G system. It is quite complicated to get set up, but very easy to use once you get it set up. If you don't have a lot of pedals, or if you don't need a lot of individual-effect control, the Gsys is a great solution. The footswitch itself is great - rugged, with nice big buttons.