I have a short commute and a fairly upright bike, so jeans work for me up to about 70 deg, after which they get too hot. I have found that some denim wears out very quickly - most of the "fashion" denim used today is much lighter than solid, classic Levi's denim.
Also, depending on your saddle, you might find that you sit right on spot in the seam where something like 8 layers of fabric are sewn together. This is a deal breaker for many!