-How powerful is this?
Mediocre. It's clearly pretty bad in money games (think Explorer), but you might want to get a with good $5 Treasures. Engines can make use of it, but it's only really good when you can play a Village beforehand (which takes some time to set up), so you don't really want to get it with your first $6.
-What are the best things to gain with this?
Seems quite nice with Horn of Plenty. It can be a super-terminal +4VP with Distant Lands. And of course there's cost reducton stuff.
-How does this compare to Altar and Count in terms of late-game Duchy gaining?
Well, if you have Artisan, there is probably a strong engine, and the game won't end in Duchy-dancing. Also you can always gain a card you can play, which will be mostly better than gaining a Duchy.
-How does this compare to Transmogrify in terms of same-turn gain-and-play?
The use is completely opposite: Transmogrify is good when you need to set up your engine, i.e. early on, and possibly really late for points and piling out. Artisan is strongest in the midgame.