Krabi Immigration: got an extension today there. Big thumbs-up to the place. Most efficient immigration I've ever seen, and the quietest (only 3 other people waiting). In and out in 20 minutes.
Here's how it works in case anyone wants to know.
1. Go to the desk where it says 'one stop service'. Tell the assistant you want an extension, then get a ticket from the machine, sit down and wait.
2. When your number is called, the assistant will ask for your address, take your photo using a webcam, and then print out your TM7. You just need to sign it - you don't need to fill it in yourself. They will also scan and print your passport, and you sign the copies.
3. Go back and sit down while the assistant gets an official to confirm and stamp your passport. Once that's done, hand over 1900 for the extension plus 100 for the 'one-stop service', and you're done.
It's so quick and efficient, it's almost worth taking a trip to Krabi or Ao Nang when it's time for your extension!
I got a motorbike taxi to immigration from the centre of Krabi Town by the way. Took about 10 minutes. 50 baht each way, or you can ask the driver to wait for an extra 50 baht (150 baht total).