When I bought my E-510 kit I kept the 40-150mm from my old E-500 kit so I could compare the two and decide which to keep.

In the end it was a close run thing. As you say, the extra speed of the older lens was pretty good, however I felt the newer lens was slightly sharper. In the end though, it was purely down to size and weight, I liked the idea of having a lighter camera bag and more space.

I felf the IS on the E-510 more than made up for the loss of speed.