Dude, I LOVE my Vectrex! I second the opinions offered already, there really is nothing like it anywhere else (except perhaps maybe Virtual Boy, without the eyestrain!)
I enjoy most every game in the library, but my favorites are: Mine Storm, Berzerk, Hyperchase, Scramble, Solar Quest, Web Wars, and the Dark Tower proto.
Just this week I got a multicart from
http://vectrex.playntradeonline.com/ for about $100 USD (Worth it for some of the rare carts alone, even though I already have 19 of the original titles CIB) and it is the way to go! Lotsa fun with the unreleased, homebrew, and proto games (Dark Tower FTW!)
Congrats on your find, and I think I have a few doubles if you are interested (Star Trek, Scramble,and Spike I believe.)
Have fun!