Federation Force. I think I'll go ahead and express my opinions.
In terms of gameplay, does it look good? Yeah, it actually does. It looks like a fun little FPS Co-op game. Don't know if I'll get it, but still, looks like an enjoyable spin-off.
Now, in terms of expectation, Federation Force is terrible. It has been 5 years since the last Metroid, and 8 years since the last GOOD Metroid. Honestly, yeah, there's nothing bad about a spin-off, except when it's been so long since the last main-series installment. When it's been that long, you would think that making a new main-series game would be their first order of business, not making a spin-off.
Now, from what I've heard, they are working on Metroid Prime 4, which will be released in 2017 on the NX. It does make me a bit happier knowing that a new main-series Metroid is on the way, but I feel it would've been better if they had announced Federation Force along with a new main-series Metroid.