I had 165s on my p-38, and I raced/toured on a trike with 155s two years ago.  Really short cranks feel funny for a while, but you getused to them really quickly.  I can't say that I ever felt any benefit from them, but I can't say I saw any performance decrease, either.  I was plenty fast on both bikes.
 
I do think, and this is just my .02 with no data to back it up, that most bent riders want to stay in the 165-170 range.  I think you run a risk of torquing the knee at 175 or above, and I haven't seen the benefit to anything below 160.  I want the crank to be long enough to get me up the hills.
 
My knees are always the first indicators of stress for me - especiallybeing arthritic as I am, which is why i was so happy to get q-rings on my tica.  That, much more than a set of cranks, cooled down the knee pains I had previously experienced, and I firmly believe that they are worth double their price in that regard.  Best money ever spent, IMO.  Much more important than a few crank millimeters.
 

Quote from FlyingLaZBoy on Mar 7^th, 2008, 10:40am:

 
 
That may be true for nancy-boys, but not for anyone lifting real weight.  Given that I usually work up to a 450 pound squat for a week a year (you can't keep it there without overtraining and risk), I know for sure that if you try to stop that load short, you will tear something.  However, go the full distance and your bones act like levers and the weight comes right back up with no strain on the joint itself.
 
Now if by "squat" you mean dipping two inches like most fools I see in the gym, I'm sure my total would be closer to 650.  
 
The idea of knee angle just doesn't apply when it's the femur and the other leg bones acting as levers to move a weight in only one plane of motion.  Totally unlike a cycling stroke.

Those are some tough lifts.  I can't even do them with my crappy grip, but some of the kids I coach can hit over 300 on a snatch, military 205, and squat well over 400.  The hormones in the beef are really doing some wonders!

exactly.  I think the best situation is the longest crank length that's right for you.  Which for most of us is 165-170.

The problem stems from the fact that these designs are essentially hollow crankarms - therefore you can't shorten them.
 
Shimano hollowtech is an example.  There isn't a real bottom bracket to speak of.  
 
Hollowtech is of the devil, IMO.  sell it off and buy a nice standard square taper or isis BB and have mark stonich make you some arms.

Got this reply from Mark Stonich, about shortening the TruVativ 175s....
 
<<<I should be able to shorten such cranks in
the near future.
 
The problem isn't the external BB but the fact that the spindle is
permanently attached to the right arm.  This is a problem with my
current fixture and standard tap holders.  I know what needs to be
made to allow shortening your 175 cranks.  First I need to
determine several critical dimensions on this new type of
cranks.  Then I need to find time to make the new tooling.
 
Finding time is a big problem....  And I am
busier than ever shortening the cranks that can be done with my
current tooling.
 
 
Mark Stonich;
  BikeSmith Design & Fabrication
    5349 Elliot Ave S.  - Minneapolis. MN 55417
        Ph. (612) 824-2372  http://bikesmithdesign.com
                    http://mnhpva.org
 
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