I just beat Gears solo. Loved it but painfully obvious it's part one of a series. Dabbled in Horde mode a little bit, it's pretty fun.
I'm about halfway through Act 3 so I should be able to finish it fairly soon. Looking forward to Horde mode as well.
I enjoyed and played the first 3, so I knew I would probably like this one and would be able to hop right in without worrying about a learning curve.