We did baby sign with both my DC. DD1 particularly took to it and had over a hundred signs. She even combined them to make specific demands (like asking to go and feed the ducks).
We never did classes. We had some children's signing books that we looked at and we just signed every time we talked. That's the key. Whenever you say food, sign food etc...
But my overriding memory of it being worth its weight in gold was when she signed at about 12 months (no older than 13 months). She had been waking at night (and I'd not been long back at work). We had tried everything- dummy, hunger, nappy etc etc. To no avail, and I was at my wits end.... then one night we were traveling home cross country, when Dd started screaming. Couldn't work out what was wrong with her, when she signed light. We turned on the car interior light, and she stopped crying, turned off the light and she started crying again. Turns out she was scared of the dark. That night, we left the landing light on, and she never woke again at night. She's now a teen, still scared of the dark, and still sleeps with the landing light on! But what baby can vocalise their fear of the dark?
We did find that initially she was slower to speak, because she signed at first. But by two her language was way, way ahead of schedule. At her two year check, she should have had 50 - 75 words, and she had over 500 (we know as she was talking part in a university research project on baby language and we had to record all of her words).
Our experience was great and I would highly recommend it!