Sonic is pretty fun, it's like Diddy Kong Racing only...a lot better actually. Like the controls are still a bit weird in air and on water but that's what it's supposed to be like. The weapons are balanced MUCH better than they were in the first game, computers are actually pretty competent and will give you a run for your money, stages look fantastic, and the overall feel is very face paced which is what a racing game should be about. I'll be able to give better impressions when I get some online racing.
Anyway, Miiverse impressions incoming. It's...extremely nice actually. The communities are really interesting and fun to look at, especially with all the fun loving artists who draw their own characters and what not. You know how you were totally expecting those things to only show up in Japanese communities? Nope. We've got some talented guys here too.
But aside from that, communities are a good example of networking done right. You can post a question or comment about something and if people are around, they'll comment. You get notifications when people comment or send you messages so you stay in the loop, plenty of people are online so someone will respond, and it's just a nice system. It's essentially a Twitter for your games. You won't write out long messages but you won't really need to.
Overall, it's one great system. I haven't really gotten a good look at it yet but once I play around with it a little more I'm sure I'll enjoy it even more.