Starts a bit slow but once it picks up, it REALLY picks up.
I'm halfway through and I've played for about 20 hours including deaths, so here are my thoughts up to this point. The game engages you for the first 5% or so, and then you do side missions for awhile so it slows down a fair amount. I started getting sick of this around 20% through, but at 25%, the storyline makes huge leaps forward. Lots of crazy stuff happens and you are launched into a new area which is much different and even more visually stunning. Once you get up to about 50% through, you'll be back to side missions and whatnot, but they will be more difficult, and you will be in a new area which makes the missions more exciting.
The variety of guns and ways you can do missions is really what makes the game great. The gun play and combat is predictable but there are so many ways you can proceed that the repetition is really not a big deal. The AI is excellent. Grenades are exactly as powerful as they should be. Enemies don't instantly run away from grenades like they do in STALKER. It often takes a ridiculous amount of hits to take down a guy which is unrealistic, but once you are used to it, you really won't care.
This game is visually stunning, exciting, and there is always plenty to do. Getting new guns and choosing the ones you carry around takes a certain amount of strategy and you can really play this game however you want to. It's fun. Really, really fun. I'm amazed that it's getting so many negative user reviews. Just play the game without any preconceptions, take it for what it is, don't compare it to anything else, get a good graphics card, and this game will blow your mind.