It depends how you play it. Some of them equate to quite tasty matches on your deposit - IF you 'cash out' the maximum allowed. I've taken ones before which have given me a 400% match. Others can be as low as 50%, so it's best to look through all the t and cs before signing up.
Others, such as Casino Rewards I believe, offer (or at least did so when I played there) the E-Z bonus release system. Basically, you could collect a high matching bonus, play it through 10% and then cash out your balance, forfeiting the remaining bonus. It works if you hit big early on.
Having said that, if for example the WR is a standard 20x (d+b) on slots, the high bonus can go against you if say it's 20x (50+200) and if the slots happen to be dry chances of busting are likely. There's nothing worse than hitting a big win that you can't withdraw and end up losing the whole lot because of the high WR. But such is the case if you take bonuses. To add to that, what with evermore restrictions regarding 'bonus abuse' and with the likely candidates of blackjack, video poker and similar low house-edge games removed from accepted games, you can find it tough to clear these.
As this has been the case, recently I've only done these to have a change from low rolling. Getting a high balance allows me to play games I wouldn't normally play, and gives the 'thrill' of high rolling in the hope of hitting it big, even if I do end up losing it all. And when I do meet the WR and cash out, it provides for a nice surprise.
I think that if you like taking bonuses and aren't concerned about losing your deposit, then these are a bit of fun. You can also test the casino out to see how the games are/how they pay/support etc. Having said that, personally I don't like these freeplays one bit. The way they are advertised makes it sound like you can play for nothing and win real money. I've yet to see this put into practice