We haven't been able to get to sing and sign classes near us but we've got the DVD and I've been signing with mine for a while now. The DVD gives you a good idea of what it's like at the classes.
Charlotte is pretty good... she can do Dog, Frog, Duck, Cat, Horse, Crocodile, Please, All Gone/Finished, No, Yes. I want to work on a few like pain, naughty, happy, sad, car, etc. now.
We also watch Mr Tumbles on CBeebies... you know that 5 min signing thing in the morning? That's really good for picking up a few signs.
Good luck. Let us know how you get on. I don't hear about too many other babies that sign.