I'd have to second the 'do it' message . I did signing with my DD and her language is amazing. She didn't really sign back until she was about 11 months, but at her peak, she had well over 100 signs. Now, she rarely signs, only if she can't make herself undrestood vocally, but at 21 months, she could say about 250 words and use 4 word sentences. She's almost 22 months now, and her word count must be well over 300 words, and she is starting to use 5 word sentences. She was assessed (for research) a while back (about 19 months) and then she had a word count over double the average for her age! She probably would have been a bit of a chatterbox to start off with, but what convinces me that signing helped was the fact that almost all of her first 100 words, were those that she had previously signed! Pretty much once she had got past mum and dad, she moved on to saying the words she could sign. When I started signing, everyone was really skeptical, but now even my very duboius MIL has been heard telling complete strangers they should do it with their babies!
I didn't have a class near me, but after a ticky start, I bought some signing baby board books off amazon and I used to show my dd the signs as we read the books. She picked that up really easily and within weeks she went from one to ten... and she just flew with it!
If you can't tell, I'm a bit fanatical about the whole thing!