OK someone asked me about these, and I lost the message so im not sure who, so I hope you find this here:
The basic idea is that you get the bonus up front but can never withdraw it, so you need to use it as leverage to get value from your early bets. There is no point just grinding the wagering, you will end up where you started, but still cant take the bonus out.
So if you put in $100 and get a $100 "sticky" bonus put all $200 on the first hand of BJ, so if you lose it is -$100. But if you win its a balnce of $400, which is +$300 on your deposit, minus the -$100 bonus they take back = +$200.
So on a Evens shot you are getting paid 2:1, thats where the value is. From there you grind out the wagering, and end up in profit.
Sticky bonuses are just as profitable as cashable ones, its just much higher variance, because you will lose half the time, and there is nothing to stop you losing 4 or 5 or 6 etc in a row, which could be bye bye £100s and £100s, so you have to have the bankroll and mental attitude to stomach the variance.