|Agents are used by many pro authors, even the best established. They relieve professional writers of the tedium of submitting their own manuscripts individually and, if any good, make each script produce more income for its author by holding out for higher rates and selling all sorts of subsidiary rights. (Apparently personal contact gets better results than sending the story in by mail.) Numerous fans have been agents, or worked for agents for a short period; in fact, it's said that you can't turn around in New York without running into a Scott Meredith graduate, and the Futurians moved into editorships from their agenting jobs.
Traditionally, ethical agents don't advertise or charge reading fees. However, many agents run (and advertise) manuscript criticism bureaus on the side, charging fees of $5 or more for this service; a few so-called agents derive practically their entire income from this source. The most prominent agents handling science fiction writers at present are Forrest J Ackerman, Harry Altshuler (both one-man operations), Ann Elmo (slightly larger, with ex-fan and editor Theron Raines handling the stf clients), Byrne and Reiss (old-time Fiction House editors), and Scott Meredith (a large outfit, and one that does have a reading fee department). Heinlein uses the services of Burton Blassingame, probably unknown outside the field but highly respected within it; Asimov was once burned by an agent who was careless with writers' money and now uses no agent at all; Tucker has an agent for his books but markets his shorts himself.
If you're not interested in crashing the proz, this is probably more than you need to know about agents. If you are, a word of advice: don't pay a reading fee. If you have anything on the ball at all, you'll find agents willing to read and criticize your work for nothing; if you haven't, some honest editor will tell you so eventually.