I have only had the 9-18 a few short weeks and have not had much experience with it. There are plenty of comments and examples in the FlickR group "OLYMPUS Zuiko Digital Club'. Look for the discussion thread dedicated to the 9-18mm. I think it was mostly this group of people that convinced me to pick it up.
I was thinking about the EX-25 myself for awhile, but have decided against it based on what I read in the Flickr Olympus groups. Have you come across the EX-25 lens compatibility data compiled by Olympus? It's a useful reference to help you judge the tube's value:
It suggests you can achieve up to a 1.32X magnification using manual focus fully zoomed to 300mm. However, (hearsay warning) that may not be much of an improvement over the bare lens. I can't seem to find the magnification stats for the bare lens. For me, I'm going to invest in a good tripod and head combo and top it off with a long lens support (Manfrotto 293). That should help ALL my shots of slow moving objects that I take with the 70-300mm (and other lenses). For me, at this point, the bare lens has plenty of macro power unassisted.