OK folks. I've been one of the biggest detractors for Verizon Messages 6.4.x, until now. Here are the solutions that I've found work to make 6.4.11 (the current version as of this writing) as reliable as 6.3.x was:
1) Chat is blazing fast, but for your own privacy and protection make sure you've chosen to NOT create a public profile that includes your name alongside your phone number. If you do this anyone with Verizon Messages can put your number in and find your name and it probably wouldn't be too hard to script the harvesting of lots of names/numbers. Once you do this, the Chat protocol, while still undocumented on encryption methods, is blazing freakin' fast. Far and away faster than SMS/MMS. It's iMessage for Android!
2) 6.4.x has a nasty tendency to crash on loading, the dreaded "Unfortunately, Message+ has stopped" message and/or just freezing when you ry to launch it. In another post, someone suggested uninstalling back to the factory image and then reinstalling the updated version solves it. I'm happy to report it absolutely seems to! You will need to reset any preferences you've changed from defaults, but you shouldn't lose any of your messages.
3) Lots of us were reporting massive delays in Androidwear notifications, from 30 minutes to hours. About an hour ago I factory reset my Moto360 Gen 2 smartwatch and allowed Androidwear to resync all my apps to my freshly re-paired watch and low-and-behold notifications are now instant! Remember, you'll have to reset any preferences you had on your smartwach too, but there are far fewer of those than if you had to factory reset your actual phone and to get instant notifications back it's well worth the time and effort.
While these seem to be a pain for the end user, and while they also shouldn't be necessary, once you do these I hope you'll find Messages is back to the stable and reliable version it was prior to 6.4.1.
Happy messaging everyone!