600's are fine to learn on, especially if you have some experience on dirt bikes. What gets people in trouble is when they are learning thinking too much about how to control the bike that they lose track of what they are doing on the road. Also think of the throttle differently than a dirt bike. You don't have to be in a powerband (2 stroke) to get the bike to move quickly. The hardest thing to get used to about street bikes is that you technically steer away from the direction you want to turn. You learn this in MSF, but on sport bikes it is way more apparent than on cruisers. So basically you push the handle bar on the inside of the turn out to turn harder. Sounds counter intuitive but trust me this is how it works...except when going slow.
In this pic I'm doing about 60mph and still have plenty of clearance to lean but you can see that my front wheel is almost straight and maybe just a hair turned toward the camera...so that takes a bit to get used to.
Also here's the bike I had before the Ape, because Motorcycle thread!