I have seen self adhesive fret markers on the internet, but I can't help thinking they will peel and break as the strings touch them as well as peel leaving a patch behind.

Just wondering if these are just a gimmick or they do work?

They work, they're very thin so they don't really fall off. Although I never tried any for several years, I usually change the design now and then. But they're pretty durable.
they last. go to ebay and look up Vinyl Inlays. the guy in Cali not "The Land Of Heavy Metal Poisoning" aka China. BTW not the good Heavy metal poisoning.

Guitars: '02 Ibanez Jem 7vwh
ESP LTD Alexi 600 Yellow
Ibanez RG560CA w/ SD Invader
Carvin DC747 Vintage Yellow
Ibanez RGA 8 QM w/ Dimarzio D-Active and PAF 8
Washburn KC-70v
Bass: 1980 Aria Pro II SB Elite II
Amps: Vox VT20+
i have those on two of my guitars from two different sellers, they are great, you can't really tell that they are stickers, and they DO NOT peel at all!
Squire Strat w/ Hot Rails
DIY "Suncaster"
Epiphone G-400
Jay Turser JT200D
Jackson JS30RR Rhoads
Ovation Balladeer
Jackson JS1 Dinky

Line 6 Spider III 75W
Behringer V-Tone GM108
Roland Cube 40