Speech therapists to help with lisping. How successful are they?

Better in children. If the palate shape and placement of the upper front teeth are correct, speech therapists can teach the brain to move the tongue to create clear sounds produced when air flows between them. It takes office time and practice at home. Children younger than five have greater problems achieving success as their palate and teeth and tongue are getting used to each other. Otherwise success is high.
Very. Very effective in children, less so in adults with "set" neuro-muscular patterns. Many people with a lisp also have a malocclusion. Normalizing the oral environment often reduces or eliminates a lisp. Ask your general dentist if an orthodontic consultation is in order.
Speech therapy is. The best way to get rid of a "permanent" lisp or, at least, reduce it greatly. Therapy that targets production of the sounds "s" & "z" at the beginning, middle & end of words differs depending on whether the lisp is frontal/interdental, lateral or palatal. A "whistling" lisp may be caused by dentures or dental appliances. See your dentist first for evaluation & appropriate referrals.

Related Questions

Is there any speech therapists or pathologists out there who can help me with lisping?

Yes and your dentist. Tongue movement in combination with the shape of the palate and the placement of the teeth in the upper arch, influences the quality of the sounds made in speech. Your dentist can evaluate the anatomical structures that the tongue uses to make sounds when air passes between them. The brain controls the movement of the tongue. Speech therapist teach the brain to move the tongue correctly.