Apologies for the late reply. I'm sure you've heard of it's terrible reviews, but it's honestly not so bad.
I've already written my review on it, it'll be up in a couple of weeks. For a brief review, it plays like a mix of Sonic the Werehog's combat for the most part. There are sections where you'll run fast, which are very fun, but they don't last too long. Shadow and Metal Sonic make extremely brief appearances, but overall I think the game was enjoyable.
Now, Sonic Boom has received some bad reviews, but instead of SEGA bailing on the whole series, I think it will only show them what exactly they need to improve on. I'm sure SEGA will treat Sonic Boom differently from the main canon, for some reason.
If you're open minded and willing to give a new Sonic a try, go for it. I got the game on the midnight release, beat it, then wrote my review all on the same day. It's not that long of a game if you put your mind to it. After you beat the game, there are mini missions you can play, or just replay the levels you've already beaten.
I'll admit, it takes getting used to. I still to this day find the gamepad a little awkward, but you'll get the hang of it after the first level, which basically acts as a tutorial.
To answer your question on getting the game for the 3DS, I won't be. Although SEGA promises an almost completely different game for the 3DS version, I'm still quite content with playing the Wii U version. ^^