Still can't find these for sale in the UK/Europe so ended up buying a 1Tb Seagate Backup Plus portable drive in silver/white (for £79.95 in store from Currys). The Seagate drive seems to work well with my MBA, one of the interesting features is the included Paragon NTFS driver (a special version that only works with this drive) that allows you to read *and* write to the drive without converting from the NTFS format its supplied in - useful, I can also connect the drive to Windows computers and even my TV (which will read NTFS drives but not HFS) to play videos etc.
The built quality of the case appears to be pretty good, though the case itself is plastic, so might not be as durable as the new G-Drives, though I bought a soft Case Logic portable HDD case for it, which fits perfectly and even has room for the cable in a front pocket, which should protect it on the move.
The USB 3.0 adaptor is also removable and can apparently be swapped for a Firewire 800 or Thunderbolt one, though these only seem to be available in black at present.
The drive itself is a 5,400rpm model and the unit itself runs very quiet for a mechanical drive. Speeds seem to be about 100Mb/s for both read/write using Blackmagic Disk Speed test, which I think is pretty good.
I might still get a 500Gb G-Drive Slim too, when it eventually goes on sale, if so, will post here on how they compare.
. Read my review of the same here - 