Upon ordering a new Macbook, I decided that the simplest option for migrating my apps and data would be a Time Machine backup (using a USB hard drive). I was aware that the new Macbook would only have USB-C ports, so also had a multi-port hub on order. Unfortunately, the Macbook arrived before the hub, so I impatiently turned it on to see what the other options were. Copying files directly from my old laptop was the obvious choice, and I didn't have any means of directly connecting the two computers so opted for a wireless transfer. I had to start the Migration Assistant on the old laptop, and this effectively shut the computer down - meaning I wouldn't be able to use it during the migration. After a few more steps (such as getting the new laptop onto the wi-fi), both computers settled down to the task in hand.
Six hours later, the old laptop was still at the "Looking for applications and data to transfer" stage, so I left them both to it and went to bed. The next morning I discovered that the power had been turned off for both laptops' chargers, and they had gone into standby. I've no idea when this happened, but turned the power on and woke up the laptops with fingers crossed. Well... the old laptop was now reporting that files were being transferred (good), but that there were 31 hours remaining (BAD). I then checked the new laptop, and it was reporting that there were 2 hours remaining - GOOD! The time remaining on the new laptop was also the only one changing, so I reckoned that the old laptop's clock hadn't updated yet. Well, sure enough, after a couple of hours it was complete, and the new Macbook worked just the same as the old one.
So to summarise, my migration involved around 210GB of data and took around 18 hours on a wi-fi connection. This would probably have been a lot faster from a USB-attached backup, but it wasn't anything like the multi-day horror stories that other people have described on the web. The other downside to this migration method is of course that both my laptops were out of action for the entire duration - and this became a bit of a worry for the hours at the start when there was no "estimated time remaining" being reported.