I have a Pentax Extension Tube K Set (you can buy this set as used around $50 on eBay). While I agree that extension tubes allow you to take really nice macro images, I must mention a few negative aspects: 1) no auto focus, 2) loss of many f-stops, 3) super thin depth of focus (DOF), and 4) tripod shooting only (due to 2 and 3).
Attached photo was taken with K100D Super + FA 50mm + Extension Tube #2&3 combined on a tripod. Due to thin DOF I needed to use f11, and with loss of much light in the tube, shutter speed I used was 1.5 second (ISO200). Well, this was taken in a relatively low-light indoor environment. So situation might be a little better outdoor or with studio lighting...