Not only is temp rating relative from person to person but also from day to day for the same person. Case in point:

In Jan. I was in the Grand Canyon and the first day was a late start, long day and late arrival at camp. Temps overnight were into the 30's but not freezing. I was chilly in my WM ultralite (the original model) even with extra clothing on. The next night it got down below freezing and I was toasty warm with less on. The difference? Sheer exhaustion the first night while I had an easy and shorter hike the next day, arrived early at camp and had a nice warm supper before going to bed.

That said, once you determine a given manufacturer's rating, you should be able to rely on it to compare how their different bags will perform for you.

We are very happy with the original Ultralite we got for my wife and recently got the newere Ultralite Super from kbennet here on this site. We also have an older WM that is very nice. I highly recommend them.