Photo Theremins

what is the deal with these guys. are they worth buying? if not how come. and what would the sound difference be between the photo theremin and a theremin with antennas

Well, as always, determining if something is "worth it" is difficult because the criteria can be different for each person.
Are they worth their cost in parts and labor? Perhaps.
Can you have fun with it and make cool sounds? Definitely.
Can they be used to make "real" music?
Not very likely.
On most I've seen you can only control the volume by turning a knob or running it through volume pedal. The biggest catch is, being that it's operated by light, it would be difficult to get the same light quality and levels from time to time and place to place with any consistancy. On stage it would be affected by the flashing stage lights unless it was shielded in a box.
BTW...I own a Professor Television optical "theremin".

Thank you for your information. I think im still gonna get one. Ill just get my money back if i dont like it. Thanks again for your post