I bought an HTC One for my mother and she loves it, especially the Zoe feature which sounds gimmicky at first but I can honestly say is one feature I would absolutely love to have on WP. The phone itself without a doubt one of the finest pieces of industrial design I've seen in a while, and there really is very very little lag, if at all. Scrolling is still not as smooth as WP, but responsiveness is stellar.
To be honest, I was quite set on getting her an 8X to transition her to WP, but she specifically asked for a gigantic screen so the 8X wouldn't cut it, while the Lumia 920 is too thick and the Ativ S doesn't have LTE.
That said, if I ever go back to Android myself, it would only be with a Nexus device. I loved Sense on my Desire, back in 2010 when stock Android was hideous and had zero social integration, but today Sense 5 is just an unnecessary skin with little annoyances that you don't notice in the store when you're playing with it but uncover over time. I still find it hard to believe that HTC's flagship is still running 4.1, even after the supply delays, when the version succeeding 4.2 is going to be unveiled in less than a month. Also, my friend's Nexus 7 is still the only Android device I've used that can come close to WP in terms of fluidity.
I played with the Galaxy S4 the other day and it was just awful. Not only does the hardware feel cheap, which I expected, but what I didn't expect from Samsung's flagship is how poorly optimized the software is. It just stutters and lags all over the place.