While I'm willing to pay for a file in specific cases that outmatch my design skills, so far I've mostly been able to make my own. I have placed an order for a specific file though, I'm really not an artist.

When you go around on shapeways or similar places there are tons of stuff you could re-design in less than 2 hours. So I really need to see the added value in the file, and most times it just isn't there. Also, it's part of the pleasure of doing things for me.

And then, there's the question of printability :
1/ When I do my own design, I KNOW it is printable.
2/ STL files are NOT the most modification-friendly format. It sucks to buy something that you can't use.

Bottom line : a real well engineered file takes time and therefore costs a lot, so most of the time, I'll be tempted to make my own instead.