MM has been absolutely horrible on my S5; which it has practically degraded to the level of a Jitterbug-- the phones for people who want phones that can't do anything. I can make and receive calls. I still have mobile data and WiFi, but no longer can use apps that require the SD card in any way, shape or form, and apps don't necessarily give you the option of altering that. OS updates are deceptively attractive because we're led to believe that we'll get to enjoy something better. We blithely assume that the vendors wouldn't spend millions of dollars hiring engineers to turn flagship and near-flagship level devices into utter junk, but they have.
In short, no. It isn't worth it. In your circumstances I'd keep my new S5--which probably works very well right now--as is. If you want something new and improved, buy yourself a more powerful phone in a year or two.