Monster opinions seem even more subjective than for most bikes, but I'll chime in here. I have a '99 M900 (w/carbs) that I've owned since new and have ridden over 30K miles. Miles ridden have been much less in the last 4 years since I bought a 650 VStrom ... which may say something. My bike is stock except K&N filter and Dynojet kit. The Monster is often tagged as a "city bike" but like other owners have said it's not great in town .... snatchy low-rpm running, limited steering lock, etc. OTOH, I have put in many 300-600 mile days and it's quite comfortable for medium-speed sport-touring (50-90 mph two lane roads). Mine has been flawlessly reliable - zero problems. Maintenance is pretty easy. Fuel economy varies a lot depending on duty cycle, but open road cruising gets over 50 mpg (US) and near 200 mile tank range.
When I go out to the garage I typically get on the VStrom and don't even think about the Monster, but when I ride the Monster on a back road and experience the smooth mid-range pull (yes, I know it's not a hotrod by any modern sport bike standards), I get a smile on my face. But the next time I go for a ride, I still get on the Suzuki. It's just an easier ride, and as I get older, I find I get more pleasure from the road and the environment than the bike itself, and less tolerant of the Ducati's idiosyncrasies. But I can't think of any reason NOT to buy one, especially as a stablemate to a GS.