The Jury's Still Out
I bought a Sargent tankbag for my 2002 RT and while it was functional, it never fit the tank, slide slightly from side to side and would activate the horn and windshield in left lock turns. When I got the GT I said enough crap, buy the bag that was designed for the bike. I waited for BMW to fix the first iteration's straps and just took delivery of my new BMW bag. But it's 20 degrees here so I won't get to use it until later this year. According to another poster here, the new version can be cinched down tight and will not activate the horn. I also like the waterproof inner bag in place of the cover I always had to stop and put on my old Sargent bag. They sell for $325 US, but Chicago BMW offers 20% off all parts and accessories.
I thought about the BigMac bag, but I don't always ride with the bag and the platen looks goofy with out the bag on it.
Have fun deciding. Oh, I believe the bag is covered under the 3 year warranty too, something you won't find with most other bags.