4.4.4 had a bit of trouble with my newly activated M8 (replacing a G3 which i was tired of waiting for updates) but i attributed it to first-day lag (when i activate any new Android phone, ever since Gingerbread, there's a period of lag and instability while it restores all my Google Play stuff and account settings) but later last night it went on and installed.
From the experience so far, and looking at reviews online, the HTC phones tend to get updates more consistently than Samsung or LG. the latter companies love giving international models all the love first, and sometimes the US (especially Verizon) never gets it and the only hope is a custom ROM (and good luck, the devs hate Verizon). When HTC rolls out updates, they all get it. I am not so sold on the metal build (coming from iPhones, and still owning a work iPhone 5S with all the dents, marks and so on) or Sense (like stepping into foreign territory--so gesture ridden and i tend to trigger them by accident) but overall i'm happy for the updates being consistent. it's a fast phone, even if a design that's dated and similar to the iPhone 5
Verizon has their own update tool, SDM.APK. it does not use the Android in-built updater that is common on Nexus and AOSP.