What I like about this version of Cortana is that she is explicitly helpful. Cortana explained that she was providing me the in-game score, and also asked midway down the page whether I wanted to continue receiving restaurant recommendations. Google does this as well, but much more impersonally.
If this is the Cortana Microsoft ships as part of Windows 10, I think you'll like it.
A series of Live Tile issues, solved with a reboot
You may experience some problems with Live Tiles on the home screen after installing the new build; I did. For whatever reason, I was able to add apps as Live Tiles soon after the build was installed. But I quickly found that I couldn't add any more apps, and that those that I had installed on the start screen either failed to show live information or inexplicably stopped working. They even rearranged themselves randomly on one occasion. That prevented apps like Cortana or my battery meter from displaying live information. Fortunately, a reboot seemed to solve the problem.
The new build also showed me two identical Search and Phone apps in my app list, leaving me to wonder which one was "wrong."
Photos continues to look better
Microsoft's Photos app has evolved into a universal app that will eventually straddle PCs, tablets, and phones, and it looks like it: The thumbnails are a bit smaller than you may be used to on Windows Phone 8.1, and they load slowly. They are, however, more neatly organized, especially with a new by-month view that allows you to quickly find your vacation photos from last August.
If you're running the new build on one of Microsoft's somewhat tired flagship phones--the 930, Icon, or 1520, as well as the new Lumia 640 or Lumia 640XL--Microsoft recommends that you download the new Lumia Camera beta app. It allows you to shoot video with Lumia Moments, then extract pictures; quickly snap pics with the camera; and dynamically adjust the flash using Rich Capture. Unfortunately, Microsoft thought I had maxed out the number of Windows phones allowed under my account and wouldn't let me download the app. Still, I've tried the new camera apps elsewhere and can wholeheartedly recommend them.
One-handed use: sort of, well, meh...
I'm a big fan of phablets, and I carry a bag that typically holds the Lumia 1520, a Lumia Icon, and a Samsung Galaxy Note 3. But on any crowded Bay Area BART train, I find it difficult to fumble with my phone while holding on to a strap or handrail. Microsoft solves this problem--sort of--via long-press of the Windows home key, which moves the top of the screen closer to your stretching fingers.
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