I have the mop that fits in the drill press or hand drill. They have many grits. I chose 220. It is great for sanding. You may remember I mentored a HS kid's project---making a desk. We sanded every thing except the top of the desk using the mop. It worked great!
We ended up with nice soft corners of the legs, stiles, etc. Left us virtually no hand sanding.
Original set up with sanding paper cost about $45.oo unless you are lucky like me. I ordered one from Sears. The delivery company said that they could not find my house and so they returned it. I called Sears. They said I would get a refund. That was about 3 months ago. With their automated phone system I could not find a correct number to press for my problem and I could not get a live person. So mine cost $90.oo because I purchased another one elsewhere.
Sandpaper refills and different grits come separately and, of course, the refills are much less expensive. However, I cannot remember the price on the refills.
It is more expensive than sheet sandpaper. However, it can do hours of sanding in minutes.
The sandpaper grit acts like it is more aggressive than the same grit on a sanding block. I am going to get another refill in something like 360 or 400.
Enjoy,
Jim