looks like good optical quality , but what i really love is how it looks on the camera ....
small elegant little lens

Gareth, The knob is for turning the focus ring.  It just make the focus ring being turned easily.  All old Leica RF lenses had that.  You must never seen a steering wheel knob neither.lol.
Voyager, It was too long ago.  I believe I spent 60 to 70 Cdn for both the Nikkor and Canon.  Still a good price.

adash,  You just push the knob and turn the focus ring to focus.  Actually, you can also hold the ring with two fingures and turn it to focus like any other cameras too.  The knob just make it easier to handle like the knob on the steering wheel.  Yes, all those old RF cameras like Leica will let the focus ring to turn a lever in the camera making the "range" finder to focus.  You know that the old RF will have double image and by turning the focus ring the images could become one for correct focus.  Well we don't need this old mechanical technology with LCD or EVF of the EP1. 

 

 

 

 
Gareth and Adash,  To show the youger generations, I am posting my collection of the Leica IIIc.  Seldom use it especially after digital.  Now those lenses are useful and I am so happy about that.  Please note that knob next to the lense.  I don't dare to use the Leica collapsable Elmar for I might forget about it and press it back to the camera to ruin my valuable Ep1 sensor.
 
 

You can mount the collapsable lens without worrying about damaging the sensor - It doesn't collapse back far enough to come anywhere near the sensor, if it doesn't foul the shutter of the leica then it shouldn't foul the sensor or shutter of the E-P1 and on top of that the diameter of the lens is bigger than the shrowd around the sensor anyway.

I have a colapsable Summar and a russian colapsable Industair 50 both mount fine without damaging anything, they can't be colapsed fully though as the rear of the colapsed shaft comes into contact with the internal shroud around the sensor. You could concievably damage the electronic lens contacts if you colapsed the lens and then wiggled it about, but its unlikely.

If your really paranoid about damaging the E-P1 you could put a collar around the lens to keep it fully extended, just a bit of cardboard taped into a loop would work.

I really like the focussing tabs on rangefinder lenses, wish I had more lenses with them on.

Jon, Now you said it, I have another lens I could try.  I was thinking about the card board collar tool.  It would help me not lossing any sleep over.
 
Gareth, Will give you a comparative photo later.

Thanks Stephen.
That explains why many RF users buy Digital PEN.

Thanks adash.  BTW, I hope I did not mislead you.  We can not turn the little knob to focus.  The knob only act as a handle to puch the focusing ring to focus.