I THINK characters have Halloween outfits all day (I've certainly seen photos from last year of characters in costumes in sunlight!), but I'm not certain.
I think the party is worth it if you love Halloween, have always wanted to dress up and/or Trick or Treat at
Disneyland, or if you weren't planning on visiting the parks on the party day anyway, since the extra-price ticket gets you up to 7 hours in the park. If you already have a park hopper for use that day and don't fall into any of the other categories, then the value of the party admission decreases somewhat, IMO.
Opinions seem to vary widely on line lengths and crowds. Some people say they were wonderfully short, while others (like myself) thought lines were much longer than expected. Even between the people I spent the party with and other members of my family who also went to the SAME party the SAME night, there were differing opinions on crowds; some of us thought it was crowded and some thought it wasn't.
So I think the experience of crowd levels hinges on a few things. It depends on you're used to (our last trip, World of Color weekend, actually, had not been crowded at all, so that's what I expected here)! It depends on expectations (I'd read all about "walking on" to rides and was disappointed by the fact that some popular rides had lines of up to and over 20 minutes...some lines seemed longer than during the day!). And it depends on perhaps having a good plan, but perhaps luck. I was the only one in my party who reads the DISboards, so in theory I was the one who had formulated a plan. And I think I was the most disappointed, because the party didn't match what I'd expected from reading here.