People increasingly care about software updates - in part because they want to get the new features, but security is becoming a more and more important topic. The Nexus devices (well, the ones from 2013 and later anyway) and Android One are a given.
Here's the scorecard for devices that are not directly backed by Google but have been confirmed to get the latest version of its mobile platform. Keep in mind that this list includes only devices that have been officially confirmed to get Android 6.0 Marshmallow. Some other devices are likely to get it too, some most likely won't, but those aren't on the list.
This is the official list as of the start of 2016, though not that some of these updates won't roll out for several months (and some are already available).