Their main thing is organized tours, with luggage van. We wanted to go on our own for the extra adventure. It's also a lot cheaper, but you have to do your own planning, find your own way around, and you cannot take home any souvenirs that don't fit in the motorcycle's luggage. For US passport holders, no visa is needed. US drivers license is fine.
Spanish drivers are generally very good.
They keep right except to pass, and they expect you to do the same. In two weeks and 3000 kms, I saw one car passing on the right on the autovia. There was almost no drifting across the lanes to straighten out the curves, which is more than I can say for some other European countries. Keep your speed down when going thru villages, and do not cross solid lines for passing. In heavy traffic scooters and narrower bikes routinely split lanes and pass on either side to get to the front.