I own the 500d and during daytime use it is really surpurb. Night time is a different matter. It does have great iso range but ive found that anything over iso 800 and a long exposure and the noise starts to get very noticible, even with long exposure noise reduction turned on. You are right about the sensors but i havent seen any noise comparisons with the 550d and the higher megapixles.
Hope that is of some help
Cheers
Scott