I agree with most of what's been said here:
My additions:
There are older 600's that are perfectly fine starter bikes
Basically 1998 and older 600's are forgiving rides that will do well with a newer rider on them.
That being said, and if you want a newer bike, I'm a big fan of the SV650, the Ninja 650 (twin) and the GS500. All are fine bikes and a lot of fun to ride. My favorite of all time is the NT650 Hawk, but good luck finding one of those in decent shape.