I've got the Sony NSZ-GT1 Bluray player / GoogleTV all-in-one-box. I bought it a year after it came out, when there was a big price drop, because I liked how it integrates with my system. The only other video source is my TiVo, which is routed through the GT1, and thus I never need to use my receiver to switch channels. It's a minor thing, but it keeps things simple.

The downside of the GT1, aside from its being discontinued and all, is that it's an older Intel platform running a no-longer-supported Android 3.x variant, while newer GoogleTV boxes run Android 4.x. "Upgrading" to a current GoogleTV box would in many respects be a downgrade, because I'd then have to use the crappy UI of my receiver to switch between GoogleTV, Bluray, and TiVo.

What one of the big Japanese AV receiver manufacturers really needs to do is port GoogleTV to run inside the receiver. Then you could have a nice interface for all the obscure features of the receiver rather than a giant remote control full of teeny little buttons. But no!

Also, it's pretty clear where the wind is blowing. Chromecast is the future, and it does almost everything I care about. The trick is whether I want to go with Plex, Avia, or something else. So far as I can tell, the Avia client works with the Plex server, so there's some ability to mix and match here.

EDIT: detailed technical discussion on xda-developers about Plex vs. Avia.
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Pondering, pondering: our circa-2008 bedroom TV only has two HDMI inputs, and three devices fighting for space there: another Bluray player, a TiVo mini, and an Xbox 360. I could swap the two Bluray players, moving the GTV box into our bedroom and using it to pass through the TiVo mini. I could then buy a Chromecast stick for downstairs. And, in theory, I could drive everything with Plex. Tempting.