Was just about to post a review. Guess here is as good as anywhere.
A little backstory though. I've been on Windows Phone since WP7. The last version of Android I used was 2.1 on an HTC Aria. Then it was to the HTC Surround (WP7), Samsung Focus Flash (WP7), Nokia Lumia 920 (WP8/8.1), Nokia Lumia 1520 (WP8.1), and then Microsoft Lumia 640 (WP8.1/W10M preview). Since I've been gone from Android so long, I'm taking a fresh, open minded look at it.
Build quality of the A9 is excellent. It feels solid in my hand without being too heavy, size is just right. Screen, Looks excellent to me, sharp and even with the brightness on a lower setting (about 2/5 setting) is still bright enough to see on a sunny day. Speaker sounds better than what I've used previously, most of the Lumias were on the back.
The OS. This is different. The level of customization is taking some getting used to, but is nice. Some of it does seem gimmicky (live wallpapers, though some of the space ones are neat), but I like the icon changeability. Notifications are similar to what I'm used to, having to disable some apps notifications as I catch them. Some of the games can be annoying in the amount they deliver. I've found some apps that keep crashing on 6.0, and I've emailed devs as I catch them. The ability to remove permissions from apps is nice. Also, I like that HTC still includes their built in flashlight app instead of having to download one.
Camera. I've only taken a couple of pictures so far, but they've been pretty good. No complaints on that front.
Battery. I know this is the hot button here. I think it's decent. If I could stop playing around on it, it would probably last. Been catching up on some games though. Plus, I'm never really that far from a charger or battery pack. If you're just using is for basic day to day use,texting, emails etc, I think you'll be fine. If you're playing games a lot, or watching a lot of video, keep a charge source handy. But then I'm coming from a Lumia 640 with a 2500mAh battery, so I'm used to this as well.
All in all, I'm pleased with the A9, and enjoying this version of Android.
Edit: A couple of things I miss from WP though. Live tiles, though some widgets are close. Glance screen. Nice being able to see if there's texts or emails without having to wake the phone fully. The notification light kind of does this, but you don't know what you have. Plus, that's overridden when charging.