I’ve been running Cyanogenmod 12.1 on my Samsung Galaxy S3 for a year or so now, and have updated to various nightly builds when they’re available. Cyanogenmod 13 became available a while back too, so I decided I’d bump up from Lollipop up to Marshmallow.
Lollipop on this one ran pretty smooth, but it looks like it’s struggling with Marshmallow, maybe it hasn’t got the oomph.
I usually find the point I start messing with custom roms is the point where I’m looking to prolong my phone just a bit longer, so maybe at this point I’m getting close to that upgrade. Still, can’t complain, this is a nearly 4 year old phone and it’s still going strong. If I use it a lot during the day then the battery will drain within a day, but other than that, it’s been an awesome phone so far.