I definitely think typing and using a mouse while standing takes some getting used to (in particular if you rest your arm on an armrest while seated); but concentration itself, to me at least, is not correlated.
I will say I think simply standing as a health benefit exists but is overblown. If you really want health benefits you have to move, so now we're talking about: treadmill desks or exercises.
My standing desk is also just high enough to fit my exercise bike under it (if I take off the handlebars), and that is really nice.
For me, using a stool makes the switch back and forth between sitting and standing much more seamless. And it's the shifting weight and switching positions that gives you most of the benefit anyway.
That said, I don't use a standing desk, I just occasionally stand up and work from a counter when I feel compelled.
It's really hard. I have had to get used to it, though, because I have yet to work in a place with enough space where I can do that.