Seems like there is little interest about this here, but I'll do a first impression anyway. In one sentence, "Its not ready for prime time."
UI - Interface is nice, and I think out of Roku, Apple TV, Amazon Fire TV and WD TV, this has the best interface. It displays what you want, feels modern, and its fairly responsive. The only thing I wonder about is how it will do usability wise when you have more apps installed. Where the UI fails is voice search. Its spotty at best. Inside some of the apps it kinda works, but one place I always expected it to work was on the home screen to either search for apps or content. Most of the time it just couldn't understand what I was saying, but in Netflix it understood most of the time. Sometimes it said it couldn't connect with Google. :\
Apps - There are almost no apps for it. Actually there are more games then video apps. Most noticeably missing is Amazon Instant Video. It comes preinstalled with Netflix and Hulu Plus, but No Instant Video, Watch ESPN, PBS, HBO, etc. And I thought you could install regular Android apps via the store, but the store actually prevents you from installing apps not specifically made for Android TV. The only version of Plex is Plex for Plex Pass, which apparently requires a paid subscription. The Youtube app isn't any better than what you find on streaming players. Its actually worse. Subscriptions are limited to some arbitrary number, which is consistent with other streaming players. Whats really annoying is when you watch a video from your subscriptions, when the video ends its starts another video right away. Sometimes its next in your feed, but mostly it seems random.
There are also maybe 8-10 music related apps, like Vevo, Pandora, and stuff I never heard of. I don't have any music or accounts with these services to test it out. I do have a bunch of music in Amazon Music, but that app isn't available either.
Remote - Its simple. A voice search button, back, play/pause, direction buttons and select. Minimal, but its fine. Its really light. Its about the size of a Roku remote, but maybe weighs 1/4 of of the weight. Its fine, though feels kinda cheap.
Other - The only redeeming thing is side loading. Its pretty straight forward. I had no experience doing this, and within 20 minutes or so after figuring out how to put it in debug mode, I was able to install the generic Plex for Android app. This is the free version that doesn't require a subscription, just a Plex account. I tried installing Amazon Instant Video for Android, but the UI was literally on its side as if it was expecting a tablet in landscape mode. Then when I went to put in my credentials it just said something like "You can't do this" and quit or crashed. But I haven't looked around to see if there is a different version I should be downloading. It gives me hope that I will eventually have a solution, and whats missing in the store at launch for Android TV can be supplemented with existing Android apps.
Conclusion - I guess my experience with the Nexus Player reinforces my opinion on Android devices in general...feels like a never ending beta. I figured since Amazon tested the waters with Fire TV, Google would build on that and make something better. It aint. If you need something now, Fire TV is not only better, its like $15 cheaper. I'll hold onto this just in case it gets better over time, but right now I wonder why Google even bothered.
Recorded Nexus Player in action.