My Turtle Beach X1's finally broke after a very long time and I've been waiting for an excuse to upgrade so went straight to the newest in their series; the X4's. I was actually thinking about picking up some Astro A40's as there's a lot of online games coming out but they were too expensive especially after you add the cost of shipping from the US.
You can get them cheaper than on Amazon if you look around stores and on eBay. I got mine for £75 as my friend works at PC World which was handy. Although £75 on a headset does seem a little too much to some, they're not just for gaming and the second reason I bought them was for watching Blu-Rays at night. They have Dolby 5.1 Surround Sound unlike my X1's and honestly sound amazing. If you play Call of Duty you will never again get killed from behind or stabbed. You can hear footsteps from everywhere and they come in extremely useful for Search and Destroy. The same goes for Halo 3 and they almost replace the radar in MLG.
As well as the audio quality, the in game chat and microphone is also of a high standard. You can alter the volume of game and chat audio to your preference and everything is crystal clear.
I noticed there is more padding and it is softer than on the X1's making them much more comfortable. My only complaint, like my friend had with his X2's is that the batteries run out fast, as they are wireless, which wasn't surprising. As long as you charge them when you're not playing, you won't notice it too much but on a couple of occassions I've had to resort to the standard free mic (and you know how I feel about that if you read my review!).
Overall, this is probably the second best gaming headset you can buy. If you play in MLG and enjoy competitve gaming then I really would recommend these but if you're a casual gamer who wants a solid headset then I'd say to get the X1's or some HD555's.