Bought the xb1 digital version, legacy adapter, and a band in a box from Costco. I actually bought an Xbox one just for Rock Band 4, because I've got every export and a full set of instruments (including my e-kit) for Xbox. I'm unhappy about all of those purchases.
The software issue with the drum kick needs to be fixed, the firmware upgrade did not fix the problem. Padding between the base and pedal helps mildly, but it's still really messy. I miss my real e-kit. I miss the keyboard. Poor filtering and sorting compared to Rock Band 3. Customization is really disappointing in how limited it is (body type, types of clothing, band logos, and so on). Calibration messes up if you minimize RB4, so I have to reset the entire console every so often. Couple of songs have big errors, like Waiting for the End being desynced by about 20 seconds or so. This is leaving aside everything that is supposed to be fixed in the December update, which frankly, it isn't really cool to say "buy the game in October so that we can fix it in December".
All in all, it's really disappointing, and I can't see it being a more enjoyable game than RB3 a year from now. Looking at the features the two games have today is just a completely sad comparison. Voting is nice. Facial hair for either gender is nice (one of my bandmate's avatar is always a woman with a beard if she has the option). The harmonies project is great. Those are the only improvements I see. I'm pretty sure I'm going to return the band in a box, because it's not worth $250 to me to have a worse version of the game I already love, in the hopes that several months from now, a few of the minor issues I have with it are fixed. I've loved Harmonix since Frequency, I've got somewhere around 1000 songs in Rock Band, and I'm hopeful that Amplitude will turn out well, as I was a first day backer.
But I regret buying Rock Band 4, and I'm pretty sure the Rock Band parties I have in the future will continue to be in Rock Band 3. Rock Band 3 plays more people, on more instruments, with more customization, and more songs. If they can get Rock Band 4's features approximately on par with Rock Band 3, I'll try it out again. But until then, it's not worth it for me. I fully understand the excitement that many here have for Rock Band 4, and I totally get that people have different feelings than I do about where Rock Band 4 stands as a platform right now and its future potential. But yeah, not happy here.