The 650's are stone reliable. They are a big bike in terms of seating position and will be very comfortable for even tall riders. It will feel lighter than the 500 Four because it's narrower and has a longer wheelbase. They aren't fast - 50 horsepower for the 1974 which is the same as the Honda 500 and 550 Fours. I've had multiple copies of each and think it would be a toss-up of which one wins the drag race or the top speed. I will give smoothness and sound to the Honda four, but the 650 Yamaha has always been one of my favorites in terms of old-school muscle. The engine makes torque and delivers it like a John Deere. From 2,000 to 7,500 - it pulls steady and will make me smile every time I short-shift the gearbox. The ratios are unique. I have never owned a bike that did it the 650 way". 1st is very tall and will go from 0 ~30 and then every gear after that is just 900 rpm away. 60 mph will put you at 4,000 rpm where the bike responds well, but 4,000 is where the mirros start getting a little blurred.
Anyway, I do like these - a lot!
Gordon