I've run the billet type adaptors as well. I was just using them as 1" spacers as I was going from 5x4.5 to 5x4.5, but same idea.
I wouldn't use cast ones, as I've seen some problems with these. The important thing to remember about the billet ones is that they are aluminum, and just like aluminum wheels you need to check the torque, but now you need to do it on BOTH sets of lug nuts. Install them, torque them properly, drive the car for 20-30 miles, re-torque them and then check their tightness periodically after that. They need to be checked more frequently than if you had no spacers and steel rims.
As long as they're torqued properly, you shouldn't have any issues. And if you're buying new rims, keep in mind you'll want to add an extra inch to whatever backspace you would normally use to keep the wheels in a similar spot.