I just tried it. Was really skeptical at first but it seems legit.
The client
is spyware/adware free. I scanned it and found nothing, and no other users have reported problems.
The client is similar to xfire. It has an ingame interface that you can disable which tracks credit earned, etc etc.
The client seems non intrusive, you can have it startup with your system to the task tray and never have to see it. Doesn't use many system resource.
You cannot simply leave your game idle and accumulate hours like xfire. If your are idle in game your status will change to idle in the client. The only way to earn points is to be constantly doing something ingame (mouse movement and keyboard strokes). They have countermeasures against auto clicking/movement programs so using them will get you banned.
The prizes seem overly expensive points wise.
There are alot of prizes. Everything from the hl2 episode pack (eps 1 + 2, hl2: deathmatch) for 9411 minutes of gameplay to high tech computer cases for 131,192 minutes of gameplay.
Shipping is included in the point cost, so the items really are free if you aquire enough credit.
The cart/checkout system is flawless, fill in your mailing address in your profile, and buying anything takes three clicks max and an invoice is sent to your e-mail.
There are a number of raffles for very expensive items that you can constantly participate in for a very small number of points (usually 100).
The mall lacks any type of advanced search or drill down function, so searching for a particular item is a hassle.
You cant buy any guild wars campaign or expansion with points from the mall.
Only about 300 supported games, so not every game will count towards earning points. Most major games (gw, wow, valve games, etc etc) are supported.
The way I see it, its like frequent flier miles. It takes a while to get something nice, but when you get something worth 300 bucks for free you realize how wise of you it was to participate. I think I will continue to run the client and eventually buy something semi-expensive.