Gambling or Bingo Cashback

Hi there, I saw some websites (e.g. Oh my dosh!) that have gambling and bingos 'cashback' opportunities. Are they good or worth the time? They usually require depositing a small amount (e.g. £5 or £10) and the cashback is 1.5 times or double the deposit amount. Is it a good use of time or are they just luring people to gamble?