There is nothing like getting a system update from one configuration of Android to another and such is the case going from Android 5.0 Lollipop to Android 6.0 Marshmallow. One of Android's greatest criticisms is the length time it takes for manufacturers and carriers to update their devices, especially older ones. If everybody ran pure Android, updating would be a much faster process, however, Samsung, HTC, LG, and to a lesser extent, Sony and Motorola and others, want to add their own UI and that is where the problems lie. The Android 6.0 Marshmallow was first introduced back in September 2015 and here it is, about 5 months later, and many devices, even flagships do not have their upgrades yet.
It now seems that Telus and Koodo are finally rolling out a workable copy of Marshmallow to the HTC One M8 and HTC One M9. In their defense, they did try to roll it out in December, but bugs caused them to stop the update until it could be fixed. While it is nice to get an upgrade, nothing is worse than getting one that messes up your device...or worse, bricks it. Reports are coming in that users on Telus and Koodo are receiving their updates - remember that the OTA (over-the-air) updates are rolled out so not everybody gets them at the same time. You can always go into settings, check for new software and 'force' the issue if you cannot wait. It is hopeful that this time around they get it right with no major bugs.
New operating systems always add new and improved power management features in an effort to help extend battery life. Marshmallow introduces us to one in particular called Doze. This allows the OS to literally suspend applications that are being used infrequently - those pesky power-consuming features that constantly run in the background draining your battery. Those applications that are not being used when your smartphone is idle with its lock screen, will Doze off. AutoSync and location sensors are two prime examples of battery draining apps that can 'Doze' off - however, pick up your device and these features will be reactivated. You will see a complete visual overhaul, especially if the manufacturer has a light UI. Rather than relying on a manufacturer to add the option - the ability to control application permissions are now baked into Marshmallow. Google Now on Tap is also part of Marshmallow and this gives you the ability to ask Google for help with whatever is showing on your display. According to HTC there will be an overhaul and improvement in the camera software and the usual "performance improvements and bug fixes" that are always part of an upgrade