Welcome on board.
You can get a lot of help here. Coming from a cruiser to a Sport Touring bike will offer you quite a different riding experience. I assume you know what you are looking for in a riding machine and if Sport Touring is your thing this a great bike.
The BMW RT's, go back quite a ways. The "contemporary" RT's stared with the R1100RTs. These were in the '98 to 2000 range. Followed by the R1150RTs then in 2005 the R1200RT, and in 2010 the R1200RT had an update with slightly different engine- a few details and a gear ratio change.
The 1100's are still running strong. Fine machines. I had two different 1100 models. They are solid. Have a bit fewer electronic gizmos but still run great. Some had a surging problem ( a mysterious increase of 250-500 or so RPMs) that was cured with a meticulous tune-up job. Some never had it some could be tuned better, some always had it and would not get better. It was a minor but sometimes noticeable thing.
The 1150's were a considerable upgrade/update over the 1100's. The also were prone to the surge issue but in late 2003/2004 there was a second spark plug per cylinder added that seemed to cure it. Some say the 1150RT were the most beautiful of the current RT models. It just looks good.
The 1200 era was a significant change. The bike got quite a bit lighter, horsepower increased, more computer controlled things and other improvements such as the servo assisted brakes made it a force to be reckoned with. It handles like a sport bike with the comfort and long-range capability of a full-on touring bike.
The 2010's introduced some subtle styling changes but mainly went to a twin cam motor. Not much change of horsepower but it did change the way the valves are adjusted--- from a screw and lock nut system to a shim under the rocker method. The first gear ratio was lowered a bit which some feel is nicer.
As for trouble years- well none of them are that way, just some "quirks" that are liked by some and hated by others. One small example in the '05's/'06's the brakes are "boosted" by a servo mechanism. This is the same idea as power brakes. The computer boosts the brake force and also allocates 60 percent power to the front brake and 40 percent to the rear. Most guys now just never use the rear brake pedal at all. Some guys do not like the servo brakes and prefer to apply all the force themselves. The computer on the 07's and beyond still allocate breaking force, just with no boost.
In the 2005/6 models the final drive was supposed to be a lifetime, no service design. This turned out to be incorrect and they are still serviceable, but with a tiny increment less convenience than in the pre-2002 and after the 2007 and beyond models. Not a big deal- they added a drain plug....
There was a rash of final drive failures, which you still find in a tiny fraction of all bikes since the 97/8's, but the change of specification of the final drive fluid volume has all but stopped the reports of failures. At worst there was something in the neighborhood of 1.5 percent +/- failure rates.
There are other who will chime-in with their own likes and dislikes for the RT's but they are great machines. They do just about everything you would want in a a sport touring bike... one that has the emphasis on the Touring side rather than the sport side of the combination. I can personally attest that the RT rill ride all day in comfort and then take the twisties with surprising strength. FJR's and ST 1300's have their strong points too but suffer from some inconveniences and flaws that the BMW does not have. Both have heat management issues and the Honda does not have a factory built cruise control. (or is it the Yamaha?)
In any case the others lack the BMW's personality which is a powerful thing.
The engines like to run, and run hard, and are designed to take it and love it. You do not want to use these machines to putt lightly around town or go bar hopping. They will do it of course but they come alive when you let them go.
Seats. There are more lines of computer chat written about the seats than just about any topic I can recall. This is complicated so I will try to keep it simple. People who come from cruisers often find the seating position- often confused with the seat itself - to be uncomfortable. Most cruiser guys find the sitting on your butt, with your feet forward to be ok, and with big fat padded seats- find that comfortable. That is not what BMW bikes are for.
They are set up to be ridden in the European style, that is to say your upper body is to lean forward some and your weight is supported by your inner thigh muscles and your upper body. as opposed to sitting on your ass. Again the seat bears a lot of the complaint, and truly the seats can be improved, but the fundamental issue is learning to sit differently. Some find the seat well designed for the purpose intended. The year of the bike seems not to be an issue here.
So, you will find a huge percentage of us came from the cruiser world, some from the big touring models like the Goldwings or the LT's... all searching for something with greater agility and less weight.
If this is what you want- you have come to the right place.
Enjoy- and let us hear your other questions.