Do they really serve any useful purpose? If so, which are good?
cleans your strings of gunk - hence making your tone more sparkley.
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Yes they do and to be honest and soft cloth will do just fine. Wipe your strings down before and after you play and make sure you wash your hands before you play. Your guitar will thank you and your tone will be better longer.
dont quote me on it, but i read that boiling your strings in water is a good way to clean them and get tone back. might do a search on it.
Dr. Stringfellow Lem-Oil. Its like $8 cleans and conditions fretboard. Will clean strings too if you wipe it off right.

Makes your neck lightning fast too.

I use it every few months or so, and use fastfret in between
Varies between people, i find i change my strings enough to get away with not having to really clean them long as i wipe em with a cloth after i play for over an hour or so
Sorry i'm late, but my walk has got terribly silly today
Sweat is one of the biggest killer of strings. Don't wipe them down? Expect rust. I just use an old t-shirt.
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GHS Fast Fret is pretty good - it cleans the crap off of your strings quite nicely. You can use rubbing alcohol for the same purpose though - just make sure to not get the alcohol on your fretboard, whereas the Fast Fret stuff won't damage it.
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