I've tried him on it a few times (I maintain his machine for him). It's always been too buggy, by which I mean, obvious visible UI bugs immediately after installation. Unity may be horrible, but it has much more robust QA testing.
Entertainingly, when I let him play with a bunch of different Linux desktop environments, the one he liked best was actually Haiku, which I threw in as a joke.
> It's always been too buggy, by which I mean, obvious visible UI bugs immediately after installation.
Really? I've installed Mate on ~10 computers, and have never run into UI bugs. I run Mate, my two kids run it on their computer, and I'm sometimes on that one helping them with school related stuff. Never run into UI bugs. I don't doubt that there are such bugs, but in my experience they're pretty rare (unless the project has taken a recent nosedive, always possible in open source).
I've run into some UI bugs using Mint Cinnamon, are you sure you were using Mate and not Cinnamon?
Do you remember what the bugs were? If it's something you can remember, I'll see if I can fix them so that future users don't run into them.
Another potential option is to throw Compiz in the mix after you have Mate up and running. Though that may make things more buggy than less, at least you can justify it with eye candy. :)
Yes, I wouldn't be surprised if Mate is slower than Xfce, since it's traditionally considered more featureful. Besides, even using a cheap $200 netbook, desktop environment speed is just not an issue for me as long as I use linux + any reasonably fast DE.
Entertainingly, when I let him play with a bunch of different Linux desktop environments, the one he liked best was actually Haiku, which I threw in as a joke.