As I see it, there are two different ways of thinking on how to play with a bonus.
The first is that the bonus can just basically pay off your house edge for a while as you gamble. You can play slots betting small without worrying about the house edge eating too much of your money in the meanwhile. You can both extend your playtime and increase your chance of coming out ahead in the end.
The other is to use the bonus as an opportunity to make larger gambles than you might otherwise. Since you're not really risking as much of your own money, you can bet bigger than you normally would, for a chance to win bigger than you would have without a bonus. Of course, you're pretty likely to lose your money this way, but once in a while you'll end up with a big win.
On the other side, bonuses also have two very significant disadvantages. As you mentioned, you can't withdraw until you meet the wagering requirement as most places, so you're forced to keep gambling, whether you want to or not.
The second is that you have to pay a lot more attention to the terms and conditions, and at some casinos you may be at a greater risk of not getting your money back.
In the end it comes down to personal preference, but if you just stick to the bonuses with reasonable terms and wagering requirements that aren't too insane, you can probably find a few bonuses out there that are worthwhile. Consider making smaller deposits when you take bonuses, so you won't feel too bad about the times you end up giving everything back trying to meet the wagering requirement.