If you look at the post by Herminator for the M Steering wheel there are pictures of the interior of the car once the new steering wheel is on - they also show the ZHP knob. I got one a week ago for two reasons:
1. I never liked the standard one (but wasn't sure about replacing with a non-standard item)
2. After I saw the pictures with it on, I had to have one - it looks completely BMW standard and a million times better than the stock item.
Also, I posted almost exactly the same question a few weeks ago!
As mentioned above, it's shorter which seems to be ergonomically better (the knob is now a little lower and seems to fall to hand more naturally) and shortens the throw a little).
You can buy them from http://leatherz.com/Merchant5/merchant.mvc?Screen=PROD&Store_Code=01&Product_Code=ZHPSSKnob&Category_Code=Knobs very fast delivery too, got mine in less than a week of ordering (which is amazing since they're in the States) - postage was $35.