I tried a 30% sour mash in crock pot my first go around which didn't provide enough souring as I would like. Plus I wasn't all about dealing with a full mash of sour grain.
Second time around I did a full mash and sparged into my brew pot, allowed that to cool to about 125 F and then into a 6 gallon glass carboy (sloooooooooooowly so thermal shock wasn't an issue). Filled all the way to the top so once the stopper was placed in there was 0 headspace. I used Wyeast Lacto which produced a lot of CO2. The stopper would actually pop out to allow the CO2 to escape so I knew I didn't have any O2 issues. Just kept it covered.
I then placed my carboy heater coupled with this cheap controller http://www.amazon.com/Hydrofarm-MTPR...ds=seed+heater and insulated heavily. Held 109-110 for around 3.5 days and had some very sour wort. This was then boiled for 15 minutes with some EKG, cooled, and pitched with Wyeast 3068. I wanted another dimension to the beer and this yeast delivered. Very clean with a slight Bav wheat flavor. Simple but one of my favorite brews and especially for the season.
I am re-using the lacto "cake" from the carboy for future sour mashes (I think of it as a "sour wort"). Carboy was bleached and then sanitized and haven't had any cross contamination issues.
The great thing about this method is the ability to obtain and then lock in a certain sourness. Though to be fair judging the exact sourness of unfermented wort is difficult because of the sweetness that counters. Plus post boil it is clean beer so my kegging and bottling equipment is unfazed.