I've had the game for a few days now and am nearing the end of the SP campaign, so I think I have a fairly good idea of what this game offers. Though I'm probably one of the last people who should be talking about a Gear game, I'll try to answer any questions you folks may have.
As for my overall opinion, here's the short of it:
Mechanically the game feels great. Guns have about the right amount of kick, and there are a ton of weapon-types to use too. I think I've sunk about 10 hours into the SP game so far, so it's pretty substantial in terms of content.
It's a gorgeous looking game on a technical level, but I have to admit that I'm just not a big fan of what they're doing with that tech. To me the Gears series has always looked too drab with textures' ham-fisted use of greys and browns caked onto every surface. It's just not my thing.
Marcus's gravely voice has always bothered me, but it seems like the voice actor has dialed up the angst for the role which reduces this character to an even more absurd level of unbelievable--even in this fiction.
However, overall, I think if someone was a fan of the original two, they will find this to be the best in the series. It really is a fun video game, and the amount of content is fantastic between SP and MP (the coop is really really well done).
I think it says something that though I'm not a big fan of the fiction, I'd still buy this game for all the gameplay/content the Gears package has to offers.