Steen's is beautiful and best ridden, in my opinion, in the fall. Less bugs, cooler temps. i used to run at a cross country training camp (in Steens) in high school and have visited the area several times. Traffic is not so bad but make sure you fill up on gas and don't expect too much in the way of services.
I don't know the best area to road bike as my experience is limited to mt. biking on the Steen's mt. loop road, which is not suitable for anything resembling a road bike. It is way hot in the summer, lots of bugs and it does cool down a bit a night.