It's harder than polished granite. And I love it.
Let's get one thing straight about this game. It is NOT FOR CASUALS! Playing on the 'easy' difficulty level is challenge enough for most gamers, but play it on 'normal' and the game really shines. Mostly because it's harder than diamonds. You WILL die many, many times while playing this game. But the game itself isn't even slightly unfair. Tactics and strategy are the key, and you'll have to make the most of all the different weapons and explore to find yourself power-ups and the like. This one is not for the impatient among us.
Flaws? Well, the graphics aren't particularly tasty, despite the lovely special effects and sheer number of enemies on some levels. And, well, that's pretty much it. So take it from me; this game is hardcore. Get it.