What is ART?
ART is the new "runtime" for Android, replacing Dalvik which Android has been using since version 1.0. What does this mean for you? Well, it's not something that is going to make you phone look or act any differently that it currently does. Android's run time is basically what turns an app's code into something the system can read and execute. Under Dalvik, your apps would need to be compiled every time you launched them. With ART, you apps are compiled when you install them, eliminating that step from your day-to-day usage of your phone.
So what's different?
Since your phone is no longer compiling on the fly, this means that apps can launch faster and your phone will use less power. In practical usage, this means that you'll see fewer delays when switching apps and your batter life should see an improvement.
Are there any downsides?
Yes. While Google provided ART as an option in Android 4.4 KitKat, they didn't think it was stable enough to make the default. I have not seen any crashes since making the move, but I've only been using it for a few days. There are some apps that have not been updated to work with ART yet as well, so visit this site to see if any of the apps you use have compatibility issues. Most apps that are currently being updated will be fine. It's typically older, abandoned apps that have issues.
Additionally, this runtime makes app updates take quite a bit longer than before. On my M8, it took about 30 seconds from the time an app download completed to when I finally got the "App Installed" notification. It's not a big deal, but you will notice it when doing multiple updates at the same time.
The good news is that you can revert your system back to Dalvik at any point. Your data is not touched at all in this process, so you won't lose anything either.
What's your suggestion, then? Should I do it on my M8?
For most people, I don't think it's necessary to switch to ART. The performance gains are noticeable, but not mind-blowing. If you want your One M8 to feel a little "snappier" or if a little extra battery life sounds good to you, then go ahead and do it. The risks are not high and if you ever want to go back, you can do it without a problem. Just make sure you check that compatibility list first and ensure none of your important apps are broken.
Ok, I want to try it. How do you do it?
NOTE: These instructions are specifically for the HTC One M8, but the directions will be basically the same no matter what phone you have.
Step 1 - Go to the Settings app and then drill down to About>Software Information>More. Once in there, tap on the "Build Number" row until you get a pop up message that says "Congratulations, you're now a developer!"
Step 2 - You will now have a "Developer options" item right above the "About" button in your Settings app. Tap it.
Step 3 - The third option down will be "Select Runtime." Tap that and you'll get this pop up:
Simply tap "Use ART" and your phone will restart. Once it's rebooted, you will have a phone running on the latest Android runtime. Congrats!
You should be aware that your phone will have to update all of your apps to complete this process, which can take some time. It took my phone about 15 minutes to complete the update, but your times may vary depending on how many apps you have installed.