(POSSIBLE SPOILERS!!!)
I've put about four or five hours into it (school/midterms this week), but I've absolutely loved it. Gameplay's solid; nothing spectacular, but it's got that Schafer charm--even though you do the same 'Ambush the guys!' mission half a dozen times, it doesn't get old because of the humor and the voicework.
The game simultaneously mocks and reveres heavy metal, and the world breathes. You can find enemies wandering around; animals attacking and killing each other; the world feels integrated and 'natural', even when green forest suddenly gives way to desert. Design's top-notch, same with the humor, although I would've expected nothing less from Tim Schafer.
The RTS sections are pretty hectic, but in a good way. I'm not a steady RTS player (I enjoy them, just I don't game a lot on my computer), so it's taking me some time to break into them, but you can still join the fray (one of my favorite things to do is drop in the middle of a crowd, bust out the Facemelter, and watch as Lionwhyte's army claws at their liquefied faces). It's a good balance of the two, although I was slightly annoyed by the fact that any time I inadvertently got close to Lionwhyte during the assault I was lightning'd to death before I could get away.
The bonuses are awesome, too. I love the 'heavy metal vistas' you can look through, the Bound Serpents to upgrade your health/attack, and--perhaps most awesomely--the Buried Metal you can find to fill out the soundtrack (I LOVE how they even explain why you have a soundtrack in the Age of Metal).
I waited two and a half years for this game, and it's everything I wanted. Damn good job, Schafer, and it's good to see you finally get the marketing push your games deserve.