Route planning on a Garmin unit can initially be frustrating until you get to grips with it. What you have planned on the computer doesn't always translate onto the device as you expect, the trick is all in the settings. Have a look in the helpful how-to's in the GPS section of the forum.
I found that running Basecamp on a Mac seems to be much smoother than on a PC, perhaps Garmin write their software on a Mac then port it across to PC ?
My old Zumo 660 is a bit long in the tooth now but still does what I need it to, but if I was in your position I would be looking at a Zumo 590LM (assuming they can be had for a decent price). The 590 screen is much better, and you can also easily share routes via bluetooth with other suitably equipped riders. With the 660 you need to mess about with microSD cards or plug into a computer to do the same.