I finally have these -- I received the headset as a birthday present and treated myself to the chatpad. I'm very pleased with both.
The chatpad isn't something you have to use a lot, but I've always hated the fiddly on-screen keyboard on the 360 (and I don't have a spare USB keyboard to hand to plug into it), so I'm happy to have an alternative. It doesn't obstruct the pad as much as I worried it would -- it's curved at the back so your third and fourth fingers can still grip the "arms" of the pad without any trouble. It's quite heavy, about as heavy as the pad itself, but after a few games I think I prefer it. I picked up a bare joypad just now and it feld weirdly light, like picking up a SixAxis after playing Xbox.
The wireless headset is a joy. I've always struggled with the wired headset, I've found it doesn't go quite loud enough for me so I have to turn the game down. Not only does this one turn up a bit louder, but it's noticeably clearer, making Wullie's accent 85% less impenetrable
Battery life is reasonable (8 hours lifetime, 6 hours to charge over USB), and it's more comfortable than the wired one. It's also more convenient if you need to walk away from the 360 to, say, get a drink or go to the loo.
Bonus marks to the headset because when you mute it it beeps at you every 30 seconds, preventing embarrassing Runcle-esque "I'm being pounced again, oh my god, why is no-one coming to save me, oh shit I'm on mute" moments. Also, you can put your finger to your ear when people talk and pretend to be Uhuru or Marcus Fenix, your choice.
I saw wireless headsets for £20 in Game on the weekend. The chatpad is £13 pre-owned from Gamestation, but remember it has the new style headset jack at the bottom, so your existing headset might not fit. Alternatively, buy a wireless headset at the same time. Both of them are luxury purchases, to be sure, but when I looked at how long I spend playing Xbox[1] they suddenly seemed a good deal more reasonable.
[1] Related: the 360 might be the first console for which I ever completely wear a controller out. Of the two I have now, one isn't too bad but the other double-taps sometimes when I pull right trigger, is absolutely filthy with hand jam, and not only are the grippy nubs on the sticks long gone but there is a thumbnail groove worn into the left one. I have no idea how I did that. Oh,