Just got my 17-40 today and it's awesome.
I'm currently using it on an aps-c body so i'm not truly getting the 17mm.
Saying that though, i went out to Kurnell last night taking shots of the moon and generaly playing round with my failing 17-85 realised that there weren't as many shots shot at 17.
I've read issues about vignetting at 17mm on FF but I guess i'll deal with that when i do go FF.
For the moment, equivalent range of 27-64 i'm quite happy with. I still am quite novice and really can't believe the clarity that i'm getting out of these lens.
(Mind you i've got nothing to compare it too )
But my wallet don't like me much these days lol.