As far as the new Material Design UI elements, I could pick at this and that, but overall I like it. The animations are cool and subtle enough to not be annoying. I like some of the colors, others suck. Not really a big deal though because several apps that I use have dark themes or can set custom colors. Hopefully those that don't soon will - Contacts, Messaging, Calendar, Keep - mostly Google stuff, I'm talkin' about you. But I gotta admit, and I didn't think I'd be saying this, I kinda like the continuity across apps that Material Design brings.
Under the hood, ART is definitely a major step forward over Dalvik. And Lollipop has been running trouble-free for me on several devices. No way I'd want to go back to KitKat. I know Lollipop has been a headache for some and you have my sincere condolences. But for me it's probably the biggest step forward Android has ever made in a single leap.
Since Android 1.0 I've seen the arguments over whether to upgrade or not. Every time a new update is released you have two groups of users:
1. Those who have to have the latest and greatest. They flash it immediately or if waiting for the OTA it never gets to them quick enough. That's OK, it's part of the fun of foolin' with phones.
2. Those who, after upgrading, hate the new version and want to downgrade. It may be for good reason, it's just not working for them. Maybe they don't like some changes, or change in general. Fine, those are legitimate reasons.
But here's the thing...
Eventually, even those who downgrade do move on. Eventually, they adapt to a newer version. Eventually, either because of hardware, software or both, they have to. Nobody seriously runs Cupcake anymore.
And they DO adapt. They get used to a new version or find ways to customize it to their liking. But some who are complaining about Lollipop now will be wishing they could go back to it after upgrading to M&M or whatever the next version is called. It happens every time.
I would rather put time and effort into adapting and moving forward than trying - and inevitably failing - to stay put. Mobile technology is moving much too fast for that to be sustainable.
Android since v1.0. Linux user since 2001.