I routinely fluff the bag before going to bed, so most of the down is on top. Enough of it then settles along the sides avoid cold spots. The coldest I've taken it without regret is the high 20s (27-28 F). I definitely feel cold, but not dangerously so, at 30 or less. I should comment that I don't bring much in the way of clothing on my trips (one base layer, one windshirt, and a nanopuff or cocoon for insulation) and I am wearing all of it when it's cold. I remember Buck Nelson recently discussing how he brought much more in the way of clothing but was able to take his sleep system much lower (like 20 degrees lower) than its rating. For me it's more weight efficient to just bring what I need in the way of clothing and take a heavier bag if cold. With my system, I couldn't take a summerlite to 20F.
edit- I should add that I don't use an enclosed tent either. This would potentially add about 10 degrees to your comfort level.