I just got a dorkfood dsv, was tempted to DIY but with two young kids I didn't have the time. First run in a slowcooker (crock pot) it maintained the temp well, and was within 1 degree or bang-on the reading from the termapen. My slowcooker wasn't very powerfull, so I picked up a 1500w hotplate which got a pot of water up to temp much faster.
I read a tip to keep the pot slightly off center of the burner which make a natural convective current due to a cold spot.
I like the potential portability of the system and that it's not a multi-tasker. Meaning I can use the hotplate/stockpot for whatever I want, then attach the controller for precise temp controlled sous-vide.