Well for a start there are quite a few limitations on where the puncture can be if it is to be repaired (how close to the shoulder, not in the wall, where on the tread, etc), and you're not allowed to have two repairs in the same quarter of the tyre. The tyre is removed, the hole is filled, and the inside is patched before the tyre is put back on the wheel. So instead of a bit of sticky string pushed through from the outside, a plug with a patch is glued from inside the tyre.
The pressure keeps the glued patch in place, the plug is part of the patch, so the plug can't come out of the tyre. It's rated for the same speed as the tyre and is expected to last as long as the tyre does. That's quite different from a string repair.
When I had one done last year, I think it cost me about £20 on a £100 tyre (those are London prices), they were busy when I dropped it off without an appointment, and I picked it up a few hours later. If they were not able to repair, they would have put on a replacement tyre instead.
Really, it's such a small amount of money over buying a string repair kit, I'd rather have a much better and safer job done by someone taking the tyre off and doing a proper patch/plug. Don't skimp on your tyres, especially with winter coming up. They are the only thing holding you on the road.