I've just purchased the new(er) Wi-Spy 2.4i and it's totally awesome.
You can see my B standard AP on channel 11 and all of the cross channel interference it would cause. I was able to see an analog Birdbox cam in science spilling right accross channels 7 to 13. so I now know to move my aps to a lower channel over there.
I over paid (£100) you can get it for £65. it's amazingly cheap for what it does.