I've used them. You can use pretty much any grade of polishing compound, but should use a different pad for each grade.
Overall they are ok, but will never give you the same results as a real buffing wheel. High speed buffers generate friction/heat much faster allowing you to slightly melt the very surface of a finish, creating almost perfectly smooth results. Of course, hold a piece stationary too long and all that friction will either create a wrinkled, semi melted blob, or even burn a hole right through the finish.
There are a lot of well written tutorials for rubbing out and polishing a finish, do some searching and a lot of reading.
The best advice I can give you for achieving best results is:
1. Surface preparation is essential - you will not get a quality finish result without starting from a quality surface.
2. Allow adequate curing/drying time
3. That's not enough time
4. Allow even a little more time
5. Do not rush anything
Ideally you should get or make a piece of scrap wood into something that approximates a guitar body/neck (ie. curves and such) and apply the same finish that you apply to the guitar at the same time you are finishing the guitar. That way you can practice your leveling/rubbing out/polishing techniques prior to doing the actual piece.
You will not do everything perfect the first time so never let the first time be your final result.
