Man I was pissed when Google bought and killed SageTV. It was probably the best DVR out there. Flexible in all the ways you'd want it to be. High Wife Approval Factor (MythTV was a disaster)
I guess they open sourced it a few years ago, but our family has moved on to just renting comcasts DVR. It's actually not that bad.
... That being said, the way we consume media has changed considerably since the days of SageTV. Youtube has almost completely taken over my TV watching time.
I did spend some time trying to get the SageTV backend working with Plex, but scheduling required an awkward web app (not wife friendly) and I could never get it naming everything in a way that made it easy to navigate with Plex. Around this time, my cable company also turned off their unencrypted QAM channels so I could no longer tune them, and that was really the nail in the coffin for me. I don't pick up enough OTA channels to make it worthwhile.
When Plex released their PVR thing, since I had the gear I set it up, but honestly, I just never watch it.
My main criteria is being able to use everything from the couch, using a single remote, and no keyboards. I was close to that with the SageTV boxes, but Google bought SageTV and it started becoming obsolete, and there was never really a good Netflix integration anyway.
I find the trouble with all PC-based stuff is switching between things and inconsistency between apps. Netflix on PC doesn't have a good way to do keyboard-based control, so remotes don't work well -- you pretty much need a mouse-style pointer. Plex works GREAT but the keyboard works differently than it does when you're using YouTube, and there's not a great way to switch. There's also all kinds of dumb problems getting multi-channel audio working with Netflix since they require some type of certified system or something.
A couple years ago I gave up on PCs and bought an Nvidia Shield, and it's basically the perfect device for me. AndroidTV is a great launcher, it supports everything I use (Plex, Netflix, Youtube, Weather, Baby cam monitor) and everything works with a single remote (I use a Logitech Harmony to control it + the TV + audio). No compatibility, rights or audio issues. And bonus: it is a Google Cast device, and also works with an OTA tuner (Silicondust HDHomerun -- though I never actually use that anymore). My TV never changes inputs: it's basically just a dumb display for the Shield, which now does 100% of content playback in my house.
It costs maybe a bit more than a nice MiniITX HTPC build, but just works (whereas an HTPC will take hours of time to get it working semi-properly).