Fast Fret does a sort of OK job, especially if you otherwise don't clean your strings or fingerboard. For optimal string sound and longevity, you'll need to do what the pros do.

Here's how to clean your frets and fingerboard the way a luthier would -

First, buy some V M, & P Naphtha, 0000 steel wool, and Tri-Flow Dry bicycle lubricant and have plenty of clean rags or paper towels on hand.

When the strings are off, scrub the frets and fingerboard vigorously with steel wool and naphtha. Give extra attention to the area right next to the frets where crud builds up.
The naphtha will dry rapidly and leave a sludge of dirt and steel on the neck.
(Don't worry about getting solvent on the finish. It'll clean it but won't hurt it.)
Remove the sludge with clean rags or paper towels and more naphtha.
Keep working with the rags/paper towels and naphtha until no more dirt is coming off.
Let it dry a few minutes.
Apply a coat of Tri-Flow Dry lubricant to the fingerboard. This leaves a teflon coating and makes the instrument feel like new.

Some warnings - use good ventilation, don't smoke while doing this and DON'T USE STEEL WOOL ON AN ELECTRIC GUITAR NECK. You'll never get the debris off the pickups. Pros use a fine grit green scratchy abrasive pad for electric guitars.

If you do this when changing strings, Fast Fret will help to keep everything working smoothly between string changes.