Mines an '01 M600, done 1400 miles in 2 1/2months ownership and been 100% reliable. Has been my commuter bike in the week and pleasure riding the weekend.
Handling is ok, nothing scary to worry about, but there's little adjustment except preload. Engine is excellent-lots of low down grunt, nice burble, air/oil cooled, 2-valve, very simple and mechanically easy to work on. Gearbox can have some big gaps between gears so you may fins some false neutrals but you rarely get them once your used to it. Engine suits the bike perfectly tho runs out of puff after 7k so short shift is best. Can cruise at 80/85mph if need be, but seems happiest from 60-80mph. Low top gear revving, 60mph=4k revs, thats 15mph per 1k revs.You can wor out the rest.
Comfortable up to around 45-50miles when seat feels uncomfortable then on, but doesn't get any worse the more miles you do. Honestly the quoted 43mpg is wrong, you will never get less than 50-55mpg. I average 62mpg overall(normally 60-70mpg commuting and weekend blasts in the country).
Belts are super easy to do, access is excellent. Reliability has been great, no gremlins, it'll happily start even after a days worth of rain with it being sat outside. Payed £1550 for mine, an '01 Dark M600, 20k miles, average condition(needed tidying) in June 2012.
I would recommend it to anyone, parts are £11 air filter, £10.80 oil filter etc. On par with yamaha, honda etc.
Overall a great bike with bundles of charactor. Has won me over big time, i used to have Fireblades and a Bandit 1200 before this but my Monster 600 has shown me there's easily as much enjoyment at the opposite end of the speed scale. Don't be put off by the low bhp output, its a light bike and does a grand job making the most of its 53bhp. Try to find a standard one, well looked after and it'll bring many years of pleasure to you!