Got my HTC One and have been playing with it since last night. I had been curious to get a WP in the past but because I've been on Verizon I would generally just get Android devices, and have ostensibly been all Android since I got the Droid Incredible when it came out. So far I really enjoy how well-integrated everything is and how smooth the experience feels.
The only real quirks I've noticed were last night I had periods where I couldn't use the store, Cortana, or Bing, and I'm not sure if it was some sort of outage going on or if it was just a bug. The podcasts app would be perfectly fine if I could add any podcast through the search, but because it has trouble finding some things I am curious to look at another one (though I believe I could manually copy-paste the RSS feeds). The only other thing is how the email accounts are separated; it feels kind of strange but maybe I'll get used to it.
Aside from the phone itself, it seems like T-Mobile has much better speeds where I live. Despite Verizon always having 3 bars, after doing a speed test I was running at 2mbps vs 25mbps on T-Mobile. I also didn't lose signal going up to work on the 9th floor like I normally do on Verizon, which is cool.