I don't own the 70-300 but I tried it at an Olympus Roadshow and I was really impressed with it, I think it's sharp even wide open. I also had the opportunity to try the 35-100 and 150 f/2 and the 70-300 still looked good even in that company. I know some people have mentioned focussing problems in low light but I think if you are aware of the limitations the image quality itself is very good.

You may have seen this review, if not it's worth a look.

http://www.wrotniak.net/photo/43/zd-070-300.html