I think it's worth going for the Christmas decorations...but I love Christmas decorations.

Main Street and New Orleans Square are particularly festive, and Toontown is very whimsically decorated.
The decorations are mostly up by the first weekend in November, and usually all up by the second weekend of November, since that's when they start the nightly holiday fireworks (Believe in Holiday Magic is a great show!). They put holiday overlays on Haunted Mansion (Haunted Mansion Holiday, with a Nightmare Before Christmas theme) and "it's a small world" (Holiday "it's a small world"), and those are incredible just by themselves. During Believe they do holiday projections on the iasw facade as well as on the castle. But you can also see the projections by themselves - they run about a 5 minute set, accompanied by music, ever 15 minutes after dark. The Christmas Parade is fun - it starts the third weekend in November if I recall correctly, and then runs every day (twice on most days) after that.
They've been doing something called "Santa's Reindeer Roundup" at Big Thunder Ranch - that's where Santa is, and they have real reindeer, and also arts and crafts (free!) for the kids. Over at DCA there is "Santa's Beach Blast",with a more laid-back southern California Hawaiian-shirt-wearing Santa. DCA is not as decorated as
Disneyland, but the Santa's Beach Blast area is nicely done.
If you're there on weekdays in the first two weeks of December crowds should be very light with short lines during the day, but the crowds will pick up at night as locals come in for the evening to see the Christmas parade and enjoy the fireworks. And weekends will be busy.
With kids...Santa's reindeer roundup is fun, and the characters in Toontown are wearing holiday outfits. The characters out in Town Square right after park opening are also wearing holiday attire, as are those in DCA at Santa's Beach Blast.