My son went once for speech therapy - both boys were due to go for specific letter sounding problems, however, by the time the appointment came round, the older one had sorted himself out. My younger son could not say the "K" sound or "G" sound. Replacing them with T and D
He was probabbly about 3 and a half at the time - the therapist was very patient with him and made it more of a game to try to assess him, but he never really went for it at the appointment - to be honest I think he was winding her up. Anyhoo, she showed us the sticky fingure technique which was to help him keep his tongue down and he was sorted out in a matter of days - he didn't need to go back after that.
i guess it will depend on the therapist, but they should be able to deal with small children and make it interesting enough to assess and work with them without resorting to the awful experience that daffy and her son had.
P.S. My son finished the appointment with a massive tantrum about something or other and the therapist did not bat an eyelid as he was jumping about screaming and banging his head of the door.
My sisters son goes for regular speech therapy as he has a problem where although he can understand what you say to him, and he can make all the required speech sounds, his brain has difficulty putting them together or remembering them. He gets extra support at school, and is learning to read in a different way to other children. He has to learn by learning each word rather than using synthetic phonics which is the norm here.
Hope he gets on okay.