I just posted a piece with photos on my blog discussing extension tubes as an alternative to macro lenses. This came about as I was considering yet another macro lens purchase and someone suggested using extension tubes instead. I picked up a used set of Kenko tubes and gave them a work out this morning.
To summarize the article -
The advantages to using extension tubes in place of a macro lens.
Small size and weight
Ability to use on more than one lens
Ability to stack tubes to increase magnification ratios
A helluva lot cheaper than another lens
Disadvantage in comparison to macro lens:
Macro use only
Very limited focal range, close-in use only
AF function has a tendency to hunt
More cumbersome, slower to use
Light loss increases with each tube size increment
If you have interest in this subject, I invite you to visit the blog:
www.foto-gizmo.blogspot.com