No experience at all, as the 50D just came out when I got my 40D (and was twice as expensive, so I didn't really consider it).

The LCD screen is said to be much better, has a bigger buffer for burst shots (although I have seldom reached the 60 pics limit on my 40), most likely better ISO management, but many people rag on it because they feel Canon rushed it out to compete with the equivalent Nikon. But this is all hearsay, no real direct experience.
As for megapixels, it's a silly race. I have a 36x24 poster on the wall shot with a 10D and 8(?)MP. No need IMO for 15MP unless you're after a very specific ultra-large application.
If you are traveling a lot and don't have laptop constantly with you, the improved screen might be worth the upgrade. There are times I can't really tell if a picture is blurry or not, supposedly not so with the 50D.

However keep in mind the next 60D (or whatever it will be called) will most likely have HD Video as well, which imo is a great plus.

Although I've done the opposite, it's usually pretty wise to save on the body, but splurge on lenses.

A great resource I find myself using all the time is Digital Camera Reviews and News: Digital Photography Review: Forums, Glossary, FAQ , their in-depth reviews are great, with sample images, comparisons, specs and everything. You may want to check it out.