Well I have used a lot of the offers. The blockbuster ones are easy and fun. Any offer that has a product that you get to try is really great, they pay good and you get to try the product. I do not mind paying a dollar or two to get a free product! Then on top of that you get $10 or $15 from Cashcrate (thats a net profit of $9 to $14 and that does not includ the product).
The reason why they pay you to try this stuff is because its a form of adversting, and it does work. For example I signed up with the Netflix offer to try it free for a month and get $20. I got paid the money but I love Netflix so much I kept using it. Well that was over two years ago and I still use netflix today

So that is why companies do this kind of adverting.
What it really comes down to is what do you care to try? What kind of offers are you care to take part in? I know a lot of people that have signed up under me really like the coffee offers and many people love the free offers (the ones where you get $0.50 for answering some questions). the amount of money you can make from it really has no end.
In the end the best thing that you can do is not to be afarid to try the big offers. Those are the ones that get approved the fastest. You may also want to get a Visa Giftcard so you can use that to pay for the shipping fee on the free products (the money you get from Cashcrate more then makes up for it). Also if you sign up with something like Netflix, be sure to get a number that you can call to cancle when you want to. Of course most let you cancle by email now anyways.
It can be a lot of fun to use Cashcrate and its a great way to actually make some money. Let me know if you have any more questions