I have one that I bought to service my GS.
There are two models of it; with Bluetooth and Without Bluetooth. The model with Bluetooth allows you to use it with some mobile OS - good if you're on tour and without a full computer. The app (I use Android) works fine but isn't pretty. The difference is $50 US, well worth it.
Then there are two versions of it, the Enthusiast and the Professional. The difference here is that the Enthusiast only allows a maximum of ten VIN numbers to receive full service functionality, but you can use it to read and clear fault codes on any number of bikes. The Professional version is unlimited. The price difference here is +$200 and an Enthusiast can be software updated to Professional.
So to answer your questions:
The fault codeq on the bike is just a light. Light goes on = Something bad has happened, please fix me.
The GS911 goes way way way beyond reading and clearing fault codes. I've never used a BMW workshop tool but I'm willing to bet that the GS911 has nearly identical capabilities. The depth of information available is huge and it also allows you to perform processes, such as bleeding air from the ABS system. You can see realtime data for pretty much every sensor, so no more guessing what's wrong. The full on PC software is well designed and very straightforward. If you're just a little bit experienced with technology and modern vehicles then you'll have no issues figuring the tool out.
I do my own services on two bikes now and I think that it will save me money in the long run and am glad to have bought it. If things go wrong (hopefully they won't) I'm confident that I can accurately and quickly diagnose the issues.
Let me know if you have specific questions... I'll try to shoot a video of it in action for you.