Hi,
For what it's worth, DS (now 3) had speech therapy from 17 mo to just under 2 and I really don't think it did anything. He enjoyed it a lot so it was positive from just the aspect of a person coming to see him (they came to our house weekly, for free, through an early intervention government program, and they were charming with him). The speech therapist was really his first "friend", which was very sweet, she was like a gushy grandma type figure and she came specifically to see him and he really liked her. She didn't do much hands on with him because he was a bit shy then, she mostly talked to me but he knew she was there to see him.
The other great outcome was it made me feel relieved (for example if he'd gotten older and never spoke and we'd never had speech therapy I would have felt like maybe I should have been doing something earlier, would have felt guilty for not beginning earlier).
Interestingly our SLP included all onomatopoeic words (like a grunt type noise to mean pig).
Before starting with the SLP I thought maybe they would have lots of things that I didn't know about. But after seeing what they were recommending I realized I had pretty much researched well beforehand and was already doing everything I should have been.
The SLP had here and there a some ideas I hadn't incorporated. She gave us a plastic mirror which was a toy we didn't have and we made that a bath toy. Another idea was to track words in a formal way, I liked doing that, making weekly lists. Another was to make photo books with his usual foods to make meal times easier and we extended the picture books to include picture-based lists of all his favorite songs etc so he could choose what to listen to(DS unlike some kids was markedly frustrated with his inability to get his points across). (As an aside when he finally did use words to pick songs he did it by number (as in "Mommy play #47 on CD 2, I guess that's a lot more succinct than saying the name of the song).
She also brought one time a tape recorder and that was great -- to play back what he'd just said was fascinating to him.
He did catch up suddenly at around 20 months and the SLP sadly refused to come anymore, too bad as he was really looking forward to her visits by then.
Polly