I have the BMW 49 Liter top case. Most of the time (e.g., daily commutes), I ride with the top case on, and the side cases off. If you read the instructions that come with the 49 liter top case, BMW recommends that you stay at 112 mph or lower.
Because you bought a new one, be aware of the clear vinyl stickers that protect the black plastic on your rack from abrasion scratches. The stickers are mentioned in the 49 liter top case manual, and apply to the K1200GT only. (Why not the others?) My dealer took the 49 liter top case out of the box and attached it to the GT when I bought it. They tossed the stickers. I only found out about them a few weeks later, after I got around to reading the manual.
Unlike some of the other people who posted on this thread, I very much feel it when the top case is mounted. Because of that, I have a love / hate relationship with the 49 liter top case.
(The following was a post of mine that I cut 'n' pasted from another thread - http://www.k-bikes.com/forums/showthread.php?t=14249
I like it because:
* It has a huge capacity; it will easily fit 2 helmets, or fit my helmet plus gloves, jacket, pants, and more. It is the only container that can hold the backpack with a 17 inch "laptop" and lots of extras inside, so I use the top case on my daily commute.
* It matches the rack on the K1200GT perfectly. The way it attaches is absolutely solid and firm.
* The handle and lock mechanisms are first class. It operates the same way as the side cases that came with the bike, only better.
* You can have the lock keyed to match the key that fits the bike and side cases, which is very convenient.
* I rarely carry passengers, but the backrest is large and comfortable. It is especially reassuring for those passengers who aren't used to riding.
I hate it because:
* It is big. I don't like the way that it catches the wind.
* It is heavy. Because sits up high and hangs off the back, I don't like what it does to the balance of the bike.
* It destroys the look and lines of the bike. I am a practical person, but even I can see the negative impact that it has on the artistic design of the bike.
* The gray top doesn't match the K1200GT - any color. Someday (soon) I will get it painted to match the bike. It is yet another expense. (I've been told that the silver color was chosen to reflect the heat. If I get it matched to the dark metallic blue of my bike, it'll be much hotter inside.)
Even though it is immensely practical, I look forward to the times when I can ride with the top case off. Still, I recommend it highly.
P.S. If you get the 49 Liter Top Case, make sure it comes with the clear plastic protective stickers. Apply them to the rack on the GT to protect the black plastic parts from marring before you attach the top case. They are mentioned in the top case manual (for the K1200GT only) but nowhere else.