I got the update 2 days ago via LG bridge. I had my G4 set up with the Google Now Launcher prior to MM, so there's really not much of a difference with the update. Here are the most noticeable ones:
When I use the Fast-Charger that came with my V10, the phone now gives a notification indicating fast charging, which it never did before. I don't know if this "enabled" fast charging, or if it just enabled the notification. Either way it's there now.
Settings menu includes Google settings, and battery life/app usage info is much more detailed. Not a gamebreaker but nice to have nonetheless.
Holding down the home button while web browsing (or doing anything really) pulls up info about whatever you're viewing. For example, if you're looking at a sports website and you hold down the home button, Google will give you some stats of some players mentioned on that page and some other stuff. I haven't found this feature particularly useful yet, but again I've only had the update 2 days so time will tell if I find more useful reasons to use it.
Easily the biggest change coming with this update is that the G4 is now enabled for T-Mobile Advanced messaging. Which at the moment is a bit useless as not a single person on my contacts list has a phone capable of it. However I did try it with my T-Mobile GS6, and it's very cool. For those that don't know, it gives you "facetime" like video calling without the use of any 3rd party app, and you can see when people are typing text messages and when they have read ones that you sent.
Battery life I can't comment on as it seems pretty similar to before. Overall, this isn't a gamebreaking update IMO.