I only have room in my garage (and time in my life) for one bike at a time. The last 10 years have looked like this for me:
2003 R1150 GS
2005 KTM 625 SXC
2006 HP2 Enduro
2007 Victory Vegas 8-Ball
2008 Triumph Scrambler
2011 Harley Softail Blackline
Yes, I change motorcycles more frequently than is fiscally responsible. I have enjoyed all of these for different reasons. Since this thread is about BMW/Harley, I will comment on that. The BMW bikes are great for travel and adventures on multiple surfaces. From a performance standpoint they do almost everything better than the Harley. But the Harley is a superbly built machine that is truly a pleasure to ride. The fit and finish is second to none. The classic design reminds me of motorcycles I admired from afar when I was a kid. I enjoy that nostalgia. Plus, being 6'6", I like the ergonomics better on a cruiser.
If I could have both, I would. However, with the present demands of a family schedule, I don't have time for the adventures that a BMW begs for. So I am presently happy commuting to work and taking short weekend jaunts on the Harley. I'm sure I'll own another BMW GS someday though. I have a dream to ride to South America.