41F here, with a 3 or 4 degree diff for the controller. Probe sits and measures freely flowing air near the bottom of the CO2 tank (around the midpoint of the freezer on the hump).
It really depends on what you're serving. If you've got IPA's, brown ales, stouts, or wheats/hefes (well, there goes a BUNCH of beers), the general consensus it to keep it around 40-45 serving temp. Allows malts and hops to really shine.
On the other hand, if you're serving pilsners, lagers, etc, then you're probably going to want mid to high 30's.
Primary #1 (SS Brew Bucket):
Primary #2 (SS 7G Chronical):
Kegged & Waiting:
ON TAP: Oatmeal Stout[br /]Holiday IPA