OP, can we swap tips? We've been going in January, February, March, April, May, July, August and December. We have no experience in October (but we will in a few weeks!) and lots of experience in January.
As for the weather, it could be anywhere from 80s to 70s to 50s. At night, it has been as low as high 30s. Sometimes it rains, and sometimes not. Our three boys often swim in January, but we do not. And those temps can vary significantly from day to night, so we either bring a backpack with extra layers or schedule a resort break to make sure we can add layers for the evening. Crowds have been relatively low for us throughout January (although we hear they are higher if a trip covers MLK weekend, and ours have not.) In any event, the weather is very unpredictable, so I highly recommend checking the forecast as you get close to your trip and packing just about everything.
In 2013, we'll be there from January 26 - February 1 and the best advice I can share is to anticipate a fun vacation, but not count on warmth. We've noticed that when people count on WDW in January as a warm weather vacation, they can be disappointed. If they count on fun and know that the weather can be unpredictable, the results are far better!
Have a great trip!!