I'm basically in the same boat as you. Plex + a box makes movie watching pretty easy. But oh back in the day HTPC was the way to go. I made a few HTPC's using slow dual core AMD procs back in 2008 when all that plus a BD-ROM drive was about the same cost as a stand-alone BR player. Set up everything with DxVA so even that pathetic hardware could play and scrub movies like a champ. Add in WMC with MediaBrowser plugin and Netflix, plus a Gyration remote = HTPC perfection. That setup is still being used today, and has the highest WAF of anything else I've tried. Wife and kids don't want to mess with mouse and keyboards, they want it to work like the cable box does. Like wagex mentioned, Plex usually has to do transcoding and can take a dump on your video or audio quality depending on your media's encoding etc.
Would I build a beefy HTPC now with fast specs? Heck no, I don't see the point. Any PC gaming I'd do on my PC infront of a monitor. Extra hardware won't help with video playback since even smartphones can play 1080p. If I was single I could see myself having a keyboard and mouse in the living room but non-tech family members just won't have it. These days I use Plex. I used to hate Plex. Why should I transcode something I worked so hard encoding in the first place damnit? Because Microsoft thats why. WMC was the only option I liked and now it's on life support. M$ doesn't want Windows in your living room, they want XBox instead. XBMC just isn't cutting it for me. Even the system I have now with updates turned off comes with its own set of maintenance that me as the tech dude in the house would have to support. My 4 year old can use Plex on the iPad better than I can.
