I am a Speech and Language Therapist and have a 15 month old son. I deliberately avoided Baby Sign classes (too much like work!) but it's an occupational hazard that I use a lot of natural gesture in my every day communication and I found that as he started to move from the prelinguistic stage to using sounds and words with meaning that I automatically started to use some basic signs with him.
His first word was "boo" for boob which was at about 11 months (though not consistent until 12). He was approximating the sign for this word earlier than this.
Here is a list of his current words expressively in speech, sign or speech and sign and in order of acquisition:
boo (11 months) speech and sign - "boo"
no (12 months) sign first, but usually uses speech. Will sometimes now wag finger for effect as well as using a conventional sign.
ball (12 months) speech first, then sign - "baw"
car (12 months) speech "ka" - sign at 14 months
drink (12 months) speech "dik"
light (12 months) sign only
finished (12 months) sign only (now says "dah" as in gone when he signs)
fish (13 months) sign first, then speech "bish"
mama (13 months) speech
dada (13 months) speech
baba (13 months) speech (used in context of "give it to me" e.g. "baba?" with outreached arm and looking at something he wants, or as a comment, e.g. looking at a picture of a baby and saying "baba")
look (13 months) speech/point "look"
bye bye (13 months) - combined speech with wave - bah bah
duck - (13 months) "duk" - started using sign today (15 months today)
this/what's this/that - (13 months) sound plus point e.g. dis? wadis? dat?
aeroplane - (13 months) - sign only has recently added sound "ush" even though we model plane and "nyaow"
train - (14 months) - started in speech as "car" plus train sign. Moved to "t t t t" with sign and is now "ch ch ch ch" and sometimes "choo"
book - (14 months) - initially word, now also using sign "book" or "bookabookabook" which means read me the book vs look at the book
tractor - (14 months) - speech with car sign - "dak" and sometimes "dakdak"
flower - (14 months) sign only
snake - (14 months) - sign and now using "sssss" as sound
crocodile - (14 months) - initially sign, now saying "nap" sometimes with sign, sometimes without
monkey - (14 months) speech first "bunk"/"book" with nasalised vowel, has recently added sign
butterfly - (14 months) - says "butt" and flaps one hand with handshape for butterfly
star - (14 months) says "car" but signs "star"
spider - (14 months) sign
lion (14 months) sign, has now added kkkkkkk as symbolic noise
bee (14 months) symbolic noise "bzzzzz" now using sign in conjunction with it
It is thought that children develop gesture and speech as separate linguistic systems prior to combining words e.g. mama there, dada juice, mama book etc. They can be used interchangeably depending on need so although conventional wisdom is that signs will be "dropped" when a word is acquired that is only true if the word is easily accessible and all the meanings associated with that word that the child wants to convey can be adequately conveyed through speech alone.
So my son still sees utility in signing because it increases his communicative repertoire e.g. he can say "car" and point to convey "car there" and may, in time, go on to say mama but sign book to show that he wants me to read a book. Many children will combine meanings using speech and gesture before they do so in words (without ever having been exposed to sign).
I don't believe that signing will confer any great advantages on my son or, indeed, that his language is "advanced" because we have used it. The RCSLT (my professional body) say there is no evidence for us to recommend it and I can see why. On the other hand, the fact he is using signs is a great source of fun and interest to me (especially as I am interested in these things anyway). I love the fact that he can communicate his thoughts in a way he might not otherwise be able to so I can "see" his thinking. Eg when we were outside a soft play last week, my sister said to him "let's go inside and
see the children playing" and he looked up into the sky and did the sign for aeroplane. If we didn't share a conventional sign for aeroplane, I wouldn't know he had "heard" her sentence in this way but I don't think that he wouldn't have thought exactly the same thing if he had never done signing. I just wouldn't know it!
I think baby signing is useful in developing parents enthusiasm for their children's early attempts at communication and developing an understanding that children can understand more than they can say but may (as in the example above) understand in a different way to what we expect! I expect it can help parents pick up on cues and follow a child's lead in communication e.g. when my son is signing "butterfly" when we read a book where they are just in the background it reminds me to follow his lead and say the word, which is something that is good for language development (conversational copresence). Arguably, however, if we paid as much attention to a point or where a child was looking it would have the same effect. Though I can't help but wonder if the fact you have "done" baby sign adds a novelty value which really reinforces this type of commenting (as well as validating it as a point is a point is a point but different signs show that your child really is learning). This "window" into a baby's learning is good fun and I have found it to be a bonding experience.
Hope this helps! There is a good section on gesture development in Eva Clark's First Language Acquisition which is quite a new book. I think she's Eva!