Hey Morrissey
I can tell you about my experiences with my '02 600. From what I've read the 650 has better power in the midrange, where the 600 is a little soft.
I still love my 600. I have had it 6 seasons and 60,000 kms. I tend to do longer trips on it, and for the past 3 years have gone to California on a 9 day, 5000 km trek. Over the 6 seasons, I slowly tweaked the bike to improve the areas that I felt would benefit:
1. Steel braided lines and EBC HH pads, helped the brakes a noticable bit
2. B12 shock and Progressive springs, lots of preload, fork height and oil adjustments. Working pretty decently now.
3. Drilled out the pilot air screws, changed shims and jets. Helped fill in the soft midrange
4. Leo Vince mid pipe and slip-on. Helped the mid and up. Sounds good too.
5. Kappa hard bags & Joe Rocket tank bag. Need lots of stuff for those long trips.
As for maintenance, I have only done the following:
a) Oil, oil filters & Air filters
b) change fork oil, brake fluid and regreased the pivots and swingarm
c) I needed a new battery, but I accidentally "cooked" my first one on my car charger
d) 2 sets of chain and sprockets. OEM only lasted 18,000 kms, the 2nd set 41,000 kms.
e) tires...
f) checked and adjusted the tappets and synchronized the carbs a couple of times.
I haven't even changed the spark plugs. I did pull them a couple of years ago to check my jetting, but they looked fine, so I put them back in. I will change them this year before I sell the bike to my brother.
After 60,000 kms, the bike has been reliable as a stone, runs better than it did when it was new, needs nothing but gas, and is certainly good to go for another 60,000 or so.
I have ridden with lots of guys with bigger and faster bikes, including B12's. The only time I have had any trouble keeping up is when the speeds exceed 180 kmph, which I don't want to do anyway, or when overtaking, when it takes the little 600 a little more time to come up to speed, even with a couple of downshifts. When the road tightens up, the 600 can easily keep up, if not lead the pack...it's noticably quicker than the B12 in the really tight stuff, and I have a lot of fun swapping lines with a buddy with a VTR1000 (SuperHawk).
One of the things I do like is that you can actually use all the power the little bike has. Vibration is pretty mild, but it is buzzier than the B12. The Water cooled 650 is likely better in this regard. That being said, I have done 1000 kms days and still been OK.
I have just put a deposit on a new Triumph ST 1050, but it's not because I'm tired of the bike, I just decided that there are a LOT of other good bikes out there, and if I'm going to have an opportunity to ride a few more in my career, then I can't keep them all for 10 years. I'm keeping the 600 in the family, as I will find it pretty hard to really part with it.
Great little bike, for what I wanted it for.