If you have an instant read thermometer and patience, you can go for it. I used the low setting on a crock pot, checking it every five minutes with my Thermopen and turning the pot on and off to regulate heat when I started playing with sous vide. I also am using ziplock bags (the freezer sort without a slide) though I have graduated to a manual air pump. I spent the money on a PID controller to handle the temperatures on the slow cooker these days. Very worth while investment. Next time I'll buy an air bubbler to keep the water moving and not layering thermally.
Right now we are finishing 25 burgers done sous vide over last evening and today for a burger bash. All cooked to within 1 degree of specified "done-ness". We'll finish by deep frying just before serving. Douglas Baldwin's tables (
online here) are my standard reference for time and temperature.
Note: Bigger water heaters (food warmer/electric roaster/large rice cooker) are coming so that the long prep time won't be quite so necessary, even for the small to medium parties held at home.