I'd never heard of sous vide before coming to this site, but all the posts about it here have made me want to try it.  I've seen the beer cooler method, which I also want to try, but, for a smaller scale test, is it possible to do it in a crock pot?  What would the differences in cooking times and techniques be? 
For cooking in the home kitchen, I find the 7 quart oval CrockPot to be an excellent vessel for sous vide when attached to an external temperature controller.  You must use the "simple" knob-controlled type (mine has low-high-warm settings available.  The models with digital controls and temperature probes are not yet programmed for sous vide cooking, although I have hopes that some day they will be.    I have used both a thermistor-based thermostat (Johnson Controls A419) and a platinum (PT-100) temperature sensor and PID controller (JLD 612 with solid-state relay) successfully.  Most do-it-yourself sous-vide home cooks like myself would assume the PID controller to be the preferred equipment, but I have found I prefer the thermostat.  The water temperature oscillates up and down about two degrees using the thermostat, and many would fear that loss of control, but my results trump those concerns.  The core of the meat remains at a stable temperature that is the average water temperature, and even very near the surface the temperature does not track the oscillations.  I use an aquarium air pump and air stones to provide circulation.    I do take control of the temperature measurement accuracy by calibrating against accurate and trusted digital thermometers.  The digital fever thermometers sold in pharmacies are accurate, and allow calibration of the thermistor or PT-100 sensor at a temperature range of about 100 F.    The CrockPot holds larger roasts quite comfortably, and with heated walls requires no separation of the meat from the walls by racks as is practiced with commercial cookers with cold walls.  I insist on using an aquarium airstone and pump to cause water to circulate, and believe this is why my results are good.  The advantages of the CrockPot as I use it over, for example, the Sous Vide Supreme, are:  1) Easier cleaning -- hard water spots dissolve away with a few drops of lemon juice, and the glass pottery can also be removed and washed in a dishwasher.  This means a lot when, as I did, the cook actually practices sous vide daily for home meals!  2) Comfortable size -- no wasted corners and cold walls means more space for larger roasts but the cooker can still be lifted without strain while filled with water.  3) Dual utility -- I can use it as a CrockPot instead of a sous vide cooker when I want to.  4) Probably the biggest advantage that the CrockPot with A419 thermostat has over PID-controller based cookers is that the thermostat recovers unattended from short interruptions in the electrical power.  The JLD612 as with most PID controllers, would allow meats cooking while I am at work to rot on the pot.  I suspect that a power outage of perhaps three seconds may result in the same problem with many commercial cookers.  The drawbacks:  1) This is an "off-label" use of the CrockPot.  I expect fairly that my CrockPot will lose a heating element to all the temperature cycling some day, though after about 500 meals it is still going strong.  Rival cannot be expected to take responsibility for such failures when their products are subjected to the unanticipated abuse. I humbly submit that the low cost of replacement of the CrockPot outweighs this risk.  I often cook roasts that cost more than the CrockPot, and don't object to regarding it as an expendable like the ZipLock freezer bags I use with it, and the aquarium tubing that I replace every few months.  2) Low prestige -- I can't show this cooker to those who've spent $450-$3000 for higher-end products and fairly expect them to understand that I'd truly prefer this to their set-up at any price.  I can't explain to the owner of a silent Sous Vide Supreme that the sound of bubbling in the CrockPot bothers no one, but means my 7-qt cooker can cook more than their 12-qt cooker, with my 85 watts bringing big roasts up to temperature faster than their 400 Watts. I can't explain to them that the 2-degree temperature oscillations from the A419 are no factor in flavor or safety, that they are at greater risk from the cold walls of their cooker than I.  I have to let them feel smug.  My final analysis: the ability to pre-cook the meat in advance of crunch time allows me, or any home cook that uses sous vide, to put dinner on the table with much less effort after I get home from work.  If I were forced to buy a high end sous vide cooker to get this benefit, it would be a small price to pay.  The tenderizing and the consistency of preparing sous vide meats has made mealtimes a great pleasure for me.  But my choice of equipment has numerous advantages that have made cooking with my setup a true pleasure.  Most of my mealtimes involve fairly familiar comfort foods that cook all day while I am at work. 24 hour tri-tip roasts, 12 hour chicken breasts -- boneless and skinless -- that I'd prefer cooking for four hours, and 24 hour pork loin come to mind. This results in the kind of mealtime convenience required for me to be able to even attempt cooking myself. If I did not have reliable regulated temperature my sous vide cooking would be limited to weekends, when I'd prefer to be eating out. I estimate I've cooked nearly 550 sous vide entrees now. I could no longer be satisfied cooking without the technique. But the beer cooler method requires constant attention that limits it's use to weekends and short cooking times. I recommend it for 35 minute fish filets, 3-4 hour chicken, or 3-4 hour beef tenderloin. Any longer cooking times will discourage its use. Now, a beer cooler with a PID controller and an electric water heater element could be good method... 