hi M.,
congratulations on your pregnancy! i started signing with my hearing baby boy when he was 5 months old. some statistics for you: he signed his first word at 7 months ("milk"), said his first word at 9 months ("hot"), signed his first sentence at 17 months ("i hear a bird singing") and is now 21 months and talking quite a bit in 2 languages. still no spoken sentences other than "i want this/ that". he may or may not be speaking in full articulate sentences at this point if i hadn't signed with him but this is something i'll never really know (and i don't think the person you know whose son was delayed in speaking really knows either -after all, we only get to see them develop once)
what i have seen, however, with my son is that he will prefer to sign something if he can't say it. it limits his frustration and enables communication. but he will also sign something and speak it at the same time (e.g. "ball") if he can speak the word. so here is where i should say that it's really important to speak the word as you sign it. would he be giving the more difficult words a shot if he didn't have the signs? i don't know. for me, it was of utmost importance that he be able to express himself and his needs to us and i can say that only once was he ever frustrated bcs of a failure to communicate... he and my sister made a sign for a certain song she sings to him(unbeknown to me) and he expected me to sing it for him when he did the sign but i was puzzled. he did it over and over and tried with my husband but no luck. finally my sister filled us in on the new vocabulary.
the other thing i've seen is that children are absolutely riveted by someone who's signing. they watch much more closely, pay more attention. it's very natural to use your hands when you talk to add exclamation to your words... kids pick up on our impulsive gestures even when we're not intentionally linking them up to words/ phrases. our son likes to purse his lips and tap on his upper lip with his finger and say "hmmm" to show that he's thinking about something... we never actively taught him this but he watches, picks up, and imitates anyway. this is true in some degree for all children.
he's now in daycare and the daycare providers don't sign but they do understand his signs (i gave them a cheat sheet). btw, you may or may not know this but boys are known to develop language skills slightly later than girls. and children who are bilingual also get the hang of language slightly later than their mono-lingual counterparts. aside from signing my son is bilingual so this may have slowed things down for him.
have a lovely pregnancy and a smooth delivery! lots of luck whatever you decide to do
a great site with a video dictionary: http://commtechlab.msu.edu/sites/aslweb/browser.htm