Not that doing a bottom end is that hard, but if you do it right, you piece of mind.
Only way I'd use a used crank is if I knew the person. Had just put a new crank in. Rode it for 20hrs, and wrecked, burned down, or was going to upgrade.
Or if I had a cheapie to flip.
Seen on another forum:
Used cranks are like used condoms, it's already someone elses mess.