GIMP 2.10 landed in Debian Testing a few weeks ago and I have to say I'm very happy about it. The last major version of GIMP (2.8) was released in 2012 and the new version fixes a lot of bugs and improved the user interface.
I've updated my Beginner's Guide to GIMP (sadly only in French) and in the process I found out a few things I thought I would share:
The default theme is Dark. Although it looks very nice in my opinion, I don't feel it's a good choice for productivity. The icon pack the theme uses is a monochrome flat 2D render and I feel it makes it hard to differentiate the icons from one another.
I would instead recommend on using the Light theme with the Color icon pack.
Single Window Mode
GIMP now enables Single Window Mode by default. That means that Dockable Dialog Windows like the Toolbar or the Layer Window cannot be moved around, but instead are locked to two docks on the right and the left of the screen.
Although you can hide and show these docks using Tab, I feel Single Window Mode is more suitable for larger screens. On my laptop, I still prefer moving the windows around as I used to do in 2.8.
You can disable Single Window Mode in the Windows tab.