As someone who owns a 450D myself, and who has a friend who owns a 500D, I would say the 500D is worth it. Functionally, there are few differences. The 450D has a 12.2mp sensor, while the 500D has a 15.1mp sensor. Not a huge difference, but enough to make a difference. The 500D supports two stops higher ISO (6400 instead of 1600...12800 is digitally enhanced, which you can do in post processing yourself). The higher ISO is a significant feature, as there are times when the 450D just can't get the shot, even with a fast lens. Another significant update to the 500D is HD video capability. That may or may not be a factor for you, depending on whether you are interested in video or not.
One of the more significant features of the 500D over the 450D is the LCD view screen. The 450D has a 230,000 dot 3" screen, which is actually fairly low resolution by todays standards, and is rather dim at times. It can be difficult to read during the day when you are out and about. The 500D has a 920,000 dot screen, which is four times as high a resolution, and considerably brighter. Using the 500D in brighter conditions is a LOT nicer than using the 450D. The higher resolution also makes it a lot easier to review your pictures in-camera, as you can see a much finer level of detail. Trying to review pictures in the 450D can be frustrating at times, as you can really never tell if your sharpness and focus is acceptable until you view your images on a computer.
All that said, the 550D is again another improvement on all of these features again, and brings some other new things into the game. The 550D has a much improved sensor with the same noise performance as the 7D. Additionally, the 550D brings full color metering to the Rebel line, which is a significant improvement over the previous metering sensors used in the 450D and 500D. The 550D also uses the Digic 4 image processor, so it is capable of outputting 14-bit RAW images, allowing for greater dynamic range and finer tonal gradations than the previous Rebels.
The price difference between a 500D and 550D is about $100, and I would say that the 550D is DEFINITELY worth it. If I had to offer up a recommendation out of all three, it would be the 550D hands down. The 500D does offer some useful improvements over the 450D, however 14-bit images, full color metering, and 50% more megapixels than the 450D make the 550D hard to beat for only $100 more.