Fantastic idea but I couldn't help but think of the Optigrab device from The Jerk.

It would be fairly simple to integrate an unobtrusive slider and eliminate the screw, solving the aesthetics dilemma.

Heavy frames with adhesive tape move over; there's new geek glasses in town!

Per Thier FAQ: "... Recent research has also shown that individuals with low-to-moderate levels of astigmatism can also achieve a good improvement in their vision after performing self-refraction with adjustable glasses. However individuals with high levels of astigmatism still require custom-made prescription lenses. ..."
So, not for everyone. Astigmatism is not too hard to check. A pattern of, uniform width, evenly spaced (all angles between lines the same angle, like every 10 or 15 degrees) lines through a point could be looked at. If all lines are uniformly sharp - then no astigmatism. If corrected for near or far sightedness which these glasses do, and lines along one particular direction are much fuzzier than lines of other directions then you have astigmatism, which varies in severity and direction from person to person.
Great fun to drag the lens in the web page illustration though. http://www.eyejusters.com/lens/
If you are over 40 and don't wear glasses for astigmatism, I think these would be great: http://www.eyejusters.com/readers/ pass cursor over "Where to Buy" may be USA only at this time. Then "click her to buy" - $40. Diopter strengths range from 0 to 4.5x *** Adjustment dials are removable and can be re-attached *** Bah: "Adjustable Readers - Due to the popularity of this item, it is temporarily out of stock. Expected to ship within 6 weeks." Also you can enter email address for updates on "adjustable reader" or "developing world". GREAT WEB PAGE IMO.

At $39.95, they're more pricey than regular reading glasses that you can get at the pharmacy.

At $40, these will be a life changer for a lot of people. Good for them.

Victor, my guess is that you don't wear glasses. Reading glasses are not regular glasses. I am near-sighted. Reading glasses don't do a thing for that. I can't pickup a set of specs without getting a doctor's visit and a prescription.
Reading glasses are all + numbers. Near-sighted prescriptions are all - numbers.
Personally, I don't see why there aren't booths for emergency specs. Walk in the booth. Dial the lenses until they focus for you, and the 3D printer makes you a set. Pay the cashier.

My vision changes during the day, don't know why (nor do docs) so I would love these.

Cheap reading glasses are the same in both eyes. Many people, like me, have different prescriptions for each eye. I just ordered a pair, waiting 6 weeks...

@ Victor E., You are absolutely correct. The benefit of these is that they are adjustable, and they adjust for each eye. The pharmacy reading glasses are fixed strengths and they are fixed at the same diopter for both eyes. I have too much astigmatism for non-prescription reading glasses including the eyejusters. The strongest clip-on reading glasses I've come across as commonly available are 3.0x. These: http://www.harborfreight.com/magnifier-head-strap-with-lights-38896.html Are may favorite of all the hood magnifiers, comes with second flop down lens inside and external loop can me moved to dominant eye, providing several strengths. You need to be closer to what you are looking at with increasing strengths of simple lenses. Also they are cheaper than drugstore reading glasses. Don't bother with the included lights, remove them.