I have been a long-time TC Electronic user, and have owned a lot of their gear over the years.
Currently, I'm re-evaluating the G-System for MusicPlayers.com since it has undergone software updates, and I'm also going to work on a tutorial for optimizing levels, using the insert loop stuff, etc.
I have the G-System being used with my Road King II and it sounds wickedly great using the four-cable wiring with the insert loop... but at the start, the sound was a serious disappointment compared to my G-Major that I previously used.
Optimizing the signal levels is the big trick, the big balancing act, and it will make or break your experience. There are Input and Output levels on the G-System to adjust, and there is the Send Level on the rear of the Road King. Once you optimize these levels, all is clean and quiet, and the effects sound great. Your Filter effects and the four pedal loops get routed to the front of your amp, while your other effects go to the effects loop of your Road King.
If you like to use multiple effects, you'll never find a solution that is 100% transparent, but that doesn't mean it kills your tone or is noisy.
The other big thing to do is follow TC Electronic's advice -- use BALANCED cables for the insert loop! They mention this numerous times in the documentation, but I'm willing to bet 75% of the guitarists complaining about noise ignore this, or don't know what it means. Go to your music store of choice and to the pro audio/recording dept. and get a set of balanced cables -- this makes a big difference because when the G-System detects balanced wiring, it changes the way audio is passed across the wire to/from your amp.
Also, if you're trying to keep your tone intact, seperate the GFX01 brain from the foot controller and put it in a small rack near your amp. If you use four 20' cable runs to/from your amp to the G-System self-contained on the floor, you will get signal loss, and this is not the fault of the G-System. You'll want to use premium quality cables for sure, and don't have them criss-cross over power cables... and if the insert loop cables aren't balanced, again... major noise issue.
When hooked up correctly, the G-System sounds amazing, but just like your Road King II and other great Boogie amps, it's not plug-and-play. You have to take time to dial these products in before they shine for you.
A lot of pro players are using the G-System now because it is so well integrated, simple to program, and offers a reasonable level of flexible configuration. Note that I said "reasonable" -- there are gearheads who need the flexibility of programming as found in the G-Force, the older Lexicon gear, etc., but most players, even ones who use lots of effects, are happy with "flexible enough" as opposed to devices that require serious study time.
As a side note, you can read a comparative review of the G-Major vs. G-Force at the link below, and we also have an in-depth review of the first-generation G-System (with a more detailed look forthcoming after they finally ship the software editor this summer):
http://www.musicplayers.com/reviews/guitars/2007/0307_TCElectronic_G.php
Scott