I think it would be a really "fun" project . . . and I see the appeal of having an "untraceable gun, but personally, I can't justify the cost with my budget.
I researched them a year or two ago, and the cost for the additional parts required to make it a fully functional pistol were more than I could afford at the time. By the time I bought the Polymer80 kit, plus a barrel, slide, trigger, sights, etc., I could get an "untraceable" gun much cheaper by just buying a used Glock from some dude in a back alley. Plus, considering my meager (almost non-existent) mechanical abilities, mine would probably be plagued with FTF and FTE issues from the jump.
I'm with you. I bought one, and when I started looking at prices for all the pieces to finish it, I probably could have bought a brand new one cheaper. Maybe if I get some used parts it will be cheaper, but then I don't know what will work and what won't work.