If you can get there a day in December before the holiday break, and on a week day, or in early December I would recommend it. The 50th celebration runs 18 months from it's original starting date of 5/5/2005. So if you really want to see the 50th stuff, make it out there before then.
Personally, after going on May 20, 2005, I did not see all the hype about the 50 celebration, sure the new parade was awesome, but it's going to be around longer than the 50th celebration from what I can understand. Fantasmic has been running for a long time, but I heard that it has improved. The gold rides were cool, but the gold paint just isn't all that wonderful in my opinion...sort of ugly-fied Dumbo if you ask me. The characters do look cool in their gold outfits, and they are out in full force, the castle is breathtaking right now. Other than that, I was almost dissappointed at some of the changes, like the mickey head floral arrangement is gone and replaced with a 50th floral arrangement that was less than great in my opinion. It was my first Disneyland trip in my life where the mickey head floral arrangement was gone, maybe I just don't like change. We had the best day at Disneyland of my life in spite of the 50th celebration, but that was because of the CM's and their royal treatment for my birthday.