So, after I actually signed up and did some reading, here are my results:

- Signing up (with my existing IGN account) worked.

- I could choose Paypal as billing option, and didn't need to link Paypal to a credit card. (My paypal account is verified through a bank account though, not sure if that makes a difference.)

- I was billed for 0$ at Paypal, also a further payment of 14.95 was announced

- According to the fine print, payment is deducted five days before the current Prime membership ends. This means: If I don't cancel the subscription within two days, I _will_ be billed for three more months.

- I immediately canceled the subscription. As described by others, this caused my "next billing" info to be set to "never". My subscription is still active for the 7 days of the trial though.

- To be sure, I also removed IGN from my list of pre-approved transactions at Paypal, as described in post #14.

- I still have access to the Prime content, could activate my pet at Gamersgate, download the free Bastion soundtrack, get free codes for Rochard and King Arthur, etc.

- I can't tell right now whether the 15% discount at GG really stays after the trial ends. I wouldn't expect it to. The IGN info page at Gamersgate (where you redeem your code to get the pet) does talk about a "permanent" discount, but no further explanations are given. It could be read as "permanent after after", or "permanent as long as you are an IGN prime member". The fact that the pet and its discount stay after canceling the IGN subscription unfortunately doesn't prove anything, since the the subscription is still in effect (even after cancellation) for the full duration of the 7-day trial.

It would be interesting to hear from someone who added his IGN pet more than week ago, whether the bonus is still applied if he buys something (the existence of the pet alone is no evidence either, since it could be present but not adding a bonus during transactions).