The multiplayer is a better and more polished version of Gears 3 with a few button control changes and better servers online (it never lags unless if my connection is bad). It's quick and easy to level-up with 3 're-up' (prestiges) to go through. The new survival mode is also pretty fun and makes a good replacement for the horde mode which began to get boring.
However, the main problem I have with this game is the campaign. I don't think I've ever enjoyed anything less (slight exaggeration). They've completely overcomplicated it with the addition of the stars that you earn based on your performance in the section (yep, chapters are now split into short sections) and the declassified missions, which is where my real problem is. The declassified missions are NOT fun (in my opinion), they're boring and teadious (especially on a higher difficulty) as some require you to run through the entire section in around 3 minutes or less and some only allow you to use certain weapons. They take away the 'fun aspect' and make it more 'arcade-y', which could be fun to some but it's not for me.
Overall the series just isn't the same, the multiplayer is awesome but the campaign is a teadious mess that is no fun whatsoever.