Google has been doing a lot of work to improve Android Studio over the last few months. The company launched the first beta of Android Studio 2.3 back in the middle of December of last year, and yesterday they they released a stable build of 2.3. This big updates adds to what many have already been testing in the beta as Google now feels the software is solid enough for it to be released into the stable channel.
The goal of Android Studio 2.3 is to focus on the quality of features that many have already been using. For example, Google made some significant changes to the Instant Run feature to make it more reliable. Pressing the Run button will now always make the application restart so that your changes are reflected in the application preview. The new Apply Changes button will do its best to actually swap the recent changes you've made in the code while keeping your application running.
The Build Cache feature was added into Android Studio with its 2.2 release, but it was disabled by default. This feature caches exploded AARs and pre-dexed external libraries which can lead to faster clean builds of your application. Build Cache is a user-wide cache that is now turned on by default in Android Studio 2.3. Another new feature is the addition of a stable release of ConstraintLayout. With ConstraintLayout, you are now able to chain two or more Android views bi-directionally to form a group in one dimension.
ConstraintLayout even supports ratios for those times when you want to maintain an aspect ratio of a widget. Speaking of widgets, the updated widget palette in the Layout Editor will now let you search, sort, and filter through your widgets for your layout. It also gives you a preview of it before dragging it onto the design surface.
Google has posted a ~4 minute summary video on what's so be sure to check that out along with the full list of changes on the Android Developer's Blog.