Just echoing a positive note for the Kenko tubes. I had a set for my old Minolta film SLR (using with a 28-85mm lens), and a set now for a 7D (usually used with a Tamron 70-300mm).
I've found the tubes don't seem to work too well with my EF-S 15-85, but with the 70-300 set to 100mm the results are pretty good.
As I do a reasonable bit of macro, I later bought the 100mm L. In comparison, I'd say the zoom with the tubes is still worth using occasionally - though be prepared to do a lot of manual focussing.
Zooming the lens (with tubes in place) will affect focus, so sometimes a slight adjustment to the focal length will help get your target in focus. A cheap focussing rail is also useful.
You may also find a tripod ring for your lens helps, as a telephoto zoom plus extension tubes can be pretty unwieldy.
Finally, even if you're thinking of later going for a macro lens, remember that the tubes are still worthwhile - adding extension tubes to a 100mm macro can get you seriously close!