I'm 5'10" with short legs (!) and originally had a 16" for a lot of the reasons given above (smaller, more manouevreable, etc.) - this was a great fit and I did all sorts of biking on it. Unfortunately frame cracked after about a year (weld on top of top tube at junction with seat tube) despite having a well fitting and long (~420mm) seat post. My LBS and Orange thankfully looked after me with the warranty and replaced and rebuilt the frame free-of-charge. However, I was advised that I had too much seatpin showing and this was likely to have been the main contributor to the crack. So, I was advised to go for an 18" replacement. I went for the bigger frame and it's been a perfect fit - no loss of 'chuckability' and probably easier to get more grip out of the front end - don't miss the loss of standover really either.
Incidentally, I've got a 16" Alpine and by playing tunes with bar width and saddle adjustment the riding positions feel quite similar.
Cheers,
Simon.