Is this a decent theremin?

http://cgi.ebay.com/Theremin-with-Volume-and-Pitch-Antenna-Best-Price_W0QQitemZ250204769225QQcmdZViewItem?hash=item250204769225

I'm really want to start playing the theremin, but I'm an extremely poor college student and can hardly find anything with pitch + volume antennas for under 150. I'm also really scared to buy an instrument online because I usually like to test them out first.

a) The pitch antenna and the volume pad are very close together, that means that moving the left hand on the volume side may affect pitch.

b) The pitch antenna isn't very thick, so that you may have bad linearity and a shortened play field.

I recommend you to spend just 58 $ more for this one:
http://search.ebay.com/b3-theremin_W0QQ_trksidZm37QQfromZR40

I also have ordered and are awaiting it. There are demo videos where Thomas Grillo demonstrates that it is really playable. So there is less risk to end up deceived.

You're asking the wrong question. It's not "is this a good theremin," but rather, "is this a good theremin for what I want to do with it".

If what you want is to make non-melodic, "free" or "noise" sounds with the theremin, or to make spooky noises with it, the jaycar will be just fine. If you want to play a melody with it I'm not sure.

There are a number of pages on this web site about how to modify it to improve it, and we can say for sure that an improved Jaycar can be used to play melodies. However, there isn't a lot of good information about how an unmodified Jaycar will perform.

I'm bidding on a B3 like Thierry suggested. still staying within' my price range which is nice :)

For me, it's still better since the US-Dollar is weak against the Euro in these days. I paid only 110 EUR, shipping included...

The B-3 is a good starter theremin. I agree that it's what you intend to DO with the theremin that will to some extent determine whether a particular theremin is a good choice.

For effects, ambient, space music, noise, etc., you can do a lot with the B-3. Its volume antenna is tricky and you may need to make an adjustment to increase the threshold between noise and silence if you intend to do precision playing - otherwise, your hand may hit the antenna a lot.

In addition, the volume loop's orientation is a bit counter-intuitive (look at the picture on the B3 web site - I've given you the URL below). It begins with a wide space where it's attached to the theremin and narrows as it extends out. I believe it would be a little easier to play if the wider portion of the loop were at the far end, more like the classic theremin design. It's just a small point.

Also, the B-3 arrives with almost nothing in the way of documentation -- I'm sure people here on Thereminworld will be more than willing to help you with any questions you have.

You can find more detailed comments about the B3 at:

www.performancekr.com/archive.html

Also, if you're located in Europe, you may want to contact Dan Burns, the B3's creator, about the power source:

http://www.soundslikeburns.com/New_Items/b3.html