[MagicRat]

On top of burly's suggestion, make sure they clean the inside of the pan itself. Its amazing all the gunky sludge (actually its eroded clutch pack particles, dirt etc) accumulates in the pan. This must be mopped out.
Also be SURE they reinstall the little magnet inside the pan. Mechanics are notorious for just throwing these things away and not replacing them. For about $1, these magnets are very cheap insurance.

Finally, some filters just have a nylon screen, (like a bug screen on your house windows but finer), but most use a fabric (Dacron) filter. The fabric kind catches much more gunk.

Oh yes, do this service on your trans every year or two. If you start noticing problems, often its too late, and your transmission is already suffering damage from old fluid or a blocked filter.