Heard of them. TBH, I can't see them being any more effective that a fine comb and certainly not as effective as the better & prescription flea treatments which deal with the flea life cycle continuously, not just adult fleas when and where you can comb.
Personally, apart from Stripe who has to have a 'spot on' wormer which also has a flea treatment (giving her tablets could be considered a dangerous sport) I rarely pre-emptively treat for fleas as basically, I haven't seen a flea, flea dirt, flea bite or any evidence of fleas in years apart from when we got Layla.
If your cat has fleas or regularly gets fleas, then use a good flea treatment like Frontline. If you have no flea problem, I would tend not to treat but groom regularly so your can spot fleas early and take appropriate action.