I bought one of these keys back in September to revitalise an old HP TC400 tablet PC. The machine already had XP Tablet Edition - basically XP Pro SP3 with handwriting recognition and a stylus input panel - but Microsoft is ending support for XP in April 2014, and I would be wary of using it online after that point. I wasn't confident of getting Linux working with the stylus and screen rotation, and the machine isn't really worth enough to waste a Windows 8 licence on it, so I opted for Windows 7. The cheapest way of getting Windows 7 installed on the machine is to download an ISO, burn it to a USB stick, and then buy a licence key from these chaps.
I assume the keys are sourced from scrapped machines that have had their Windows 7 OEM installation deactivated. I mention this because after using the key (which was emailed to me within five minutes) I had to phone up Microsoft and interact with an automated telephone assistant - this involved typing a bunch of six-digit numbers into the phone and then reading off a bunch of six-digit numbers and plugging them into Windows 7. My guess is that the key had been first activated on another machine which had subsequently been stripped for parts. If I'm correct there's a risk that they might try selling the same keys more than once, time will tell.
After activating the key, Windows 7 shows up as a legitimate, activated installation. I can use Windows Update, and it continued working after I replaced the existing hard drive with an SSD, which involved cloning the OS onto the SSD and swapping hard drives. After two months it continues to work.
Windows 7 Home Premium itself is basically all the Windows 7 you need - I have a boxed copy of Windows 7 Pro which I bought entirely for the XP virtual machine, but beyond that the Pro (and Ultimate) additions can be achieved by other means.