I have an Ixus 500 too. A couple of people at work have Ixus 40's and 50's. They seem really good, nice big screen and seemingly comparable performance. The only thing for me is they take SD cards instead of CF, I've got loads of CF so it would be a bit of a pain to swap. Other than that I'd probably have bought a 50 had they been about when I bought my 500.

That said the Ixus Wireless looks interesting.