I did it. Best project I ever undertook on my bungalow. Found the sashes at junkyards, salvage places, Craigslist. You can learn to resize sash, as well, so that's not as crucial -- surprisingly to me, I found that the widths are usually standardized (24", 31.5", 34", etc) and if your house is from the 20th century, this is likely to be the case. If you're lucky, the replacement gorillas left the pulleys intact, so all you'll need are a good supply of Sampson spot cord and lots of new parting bead from a good lumberyard.
The white vinyl windows we inherited, which looked dreadful once we'd stripped all the trim down to the original fir:
Removed:
The frame and pulley holes:
Almost done:
Done!
A comparison, for anyone who doubts the advantages:
Q