I was torn between getting the Nano Lite Pro shorts or this, and decided to pick this because of the better chamois (my Aero Race already has the same one, and it's fantastic) and purported knee coverage. I've been through a recent rain, and find that being wet doesn't feel uncomfortable or cold; giving up the water protection of the nano shorts wasn't a big deal.
Well I don't know if I made the right choice or not, but the Omloop sure is a fantastic choice nonetheless. First off, the length. The shorts extend just over the knee, with a soft fleecy material that keeps in the warmth. It's elastic, but it's a very soft grab. There's a little bit of extra material when standing straight up, but I didn't notice the wind flapping it about when riding. The legs didn't ride up whatsoever.
The fit is surprisingly true. My only other Castelli shorts were the Aero Race, and those were a little on the compressive side. These just like to hug your body in the soft material and feel like a second skin. Same size for both, obviously.
As for warmth, well going by Castelli's website, these are warmer than the nano shorts, given the extra coverage on the knee. I took these out on a windy ~65 degree ride; a bit warm for tights or leg warmers, but perfectly fine for these. Plus, if it gets windier/colder, just slap on the warmers.
Given the flexibility of these shorts, I'd recommend giving these a try. Sure, you might get more wet on those fall rides, but the comfort of the better chamois and knee coverage is a godsend.