1) Maximiles change the value of their points without notice, so my review may be out of date by the time you read this.

2) Spend money: However, as of Jan 2011: You need 3500 points to get a DVD (Toy Story 3 = cost around £10) I got 45 points for spending £58 at Asda online grocery. So, around 1 point per £1 spent.

***Therefore to earn the 3500 points needed for a single DVD, you would need to spend around £3500

3) Spend time: I read from other reviews, that you can earn 10 points for filling out a survey that takes around 3minutes. So that's 3.33 points per minute. If each point is worth around 1/3500 of a £10 DVD, I calculate that means you earn around 1p of DVD per minute of your time spent filling in surveys.

4) Not only are you being paid just 1p or so per minute of your time, but, if you answer honestly, you are giving away details of your shopping habits and preferences, which will no doubt be used to target you all the better, and persuade you to buy -more-, not save more.

Seems to me that with the time you spend signing up, reading email newsletters, and deciding what to save for, then checking and rechecking your points score (and their current market value), you are actually losing out.
You lose time, lose privacy, and for what? For 1p per minute? For 60p an hour? Stuffing envelopes pays more, and you don't have to give up personal info that may well end in you spending more (because you spend so much more time reading/viewing products).

Very clever marketing going on here. Nothing else though imho. I hope that I have totally miscalculated due to being on Morphine at the moment. But possibly not....