I saw King's Bounty Platinum for $5 and might buy it, but I wanted to ask gamers here if they have any experience with Amazon game downloads and whether its any good or not and how reliable is the service. Thanks.

All I can say is that I personally have never had any problems. The speeds are decent and the download page is always available, but the game forums there are pretty meh-ish and I'm not sure how specialized the support is if you do have an issue.

I only bought one, Alice:Madness Returns, which registered on Origin anyway. They key took a couple hours to register post-purchase, however, which made no sense to me.

I've only downloaded Homefront on release day from there and it forced me to d/l the setup files completely before it would show me the cd-key needed to register on Steam.

I've never had a problem. It works quite well for me.