The reordering feature looks exactly like how I'd want it. It's a feature I haven't found in any other client. I've stuck with gitk until now, but this might just make me take a look at GitHub Desktop.
Show log
Select a commit, 3rd or older
Right click
Rebase ... onto this
Check 'Force Rebase'
Drag and drop to reorder
Start Rebase
It still seems like a pretty obscure feature though. I believe none of my colleagues exploits this or even see a reason for why they may want to do so.