Our DD started signing at about a year - we had been doing certain signs with her since birth: change (diaper), eat, more, and finished.
It does cut down on the frustrations like everyone said. Then there are moments like 2 minutes ago when DH had signed with her that he was going to change her diaper, and then she kept signing finished to me- we finally decided she was saying she was finished playing.
She started talking before she started signing - she said Dada, Mama, Da (dog) and a couple of other things before she started the signing. She is still actively working on her words, but in the mean time, given that she is a girl who knows her own mind, it is nice that she can express herself without going "AAAAH" quite as much of the time.

MTA: DH actually speaks Swiss German with DD and I speak English with her. It is interesting. Each of us can say a word that she knows a sign for in the respective language and she does the corresponding sign. I think they are just little sponges at this age and they just soak it all up and end up with fluency in more than one language. That's what happened with DH - his dad is British, his mom Swiss and he just grew up speaking both languages. I think it will be the same for these kids who learn to sign as well as speak.