I like the 26x1 3/8" (590mm) wheel size because I have short legs. I have a compact geometry hybrid with 700c wheels that fits ok, but classic geometry/horizontal toptube isn't an option. There are plenty of 700c road bikes made for riders with legs as short or shorter than mine (31.5" bike inseam, 29" pants inseam), but they're made for short riders. I'm 6'1".
Some of the old 26x1 3/8" bikes have good standover height and a good toptube length. With a longer stem I get a better fit on an old 26" road bike than on almost any new bike (excluding custom).
The one I've been riding for the past couple years also has 165mm crank arms (much more common one these bikes, if you don't mind one piece cranks) which makes it MUCH easier for me to maintain a high cadence.

A while back there had been a push for 650b wheels for touring and commuter bikes. If you're interested in the advantages of smaller wheels and converting to smaller wheels do a search for 650b.

My only problem with 650b is cost (which, for good quality, is also an issue with 650a) and the fact that 650a was already a more pervasive standard (for older bikes and wheelchairs).