I had a FABULOUS time! I am already planning on going back in the next couple of years! There were lines and it was crowded, but mostly it wasn't noticed. I think the longest we waited was maybe 50 minutes...? But they keep the lines moving very well, so really it doesn't seem quite that long!
My advice/thoughts:
START WALKING! That is my first piece of advice! I walked about 9 miles everyday while there! I really wanted to stay later, but by the end of the day, my feet and legs were giving out.
If you are there for a few days and don't have to try to squeeze everything in, go in the morning, go back to the hotel for lunch and a rest, then go back for the evening. Park gets super busy during midday and if you have little ones, the break is a necessity!
Have a game plan when it comes to bouncing back and forth to the parks (if you have a park hopper) and plan out the Fast Passes as much as you can.
If Peter Pan has a wait less than 45 minutes GET IN LINE! That is one of the rides that we never got to get on because it ALWAYS had 50 minutes or more!
If you have a little one, do one of the character breakfasts. My daughter and I got to do the princess breakfast and the HUGE price was worth it. I have some really great pictures of her and the princesses and they signed her little autograph book and she LOVED it!
GET THE DISNEYLAND APP. You can look up wait time, see what times the characters come out, etc. Knowing the characters come out is important, because if you want to see them, you pretty much have to be there when they come out.
Take some moleskin to cover up blisters! Pretty sure most of us ended up using it at one point or another!
Buy a flat folding water bottle and fill it up throughout the park. MUCH cheaper than the $5 water bottles in the park.
Whew! Those are the big tips I have for now. I'll check back if I think of anything else! Most of all, have fun with it.
Oh, and get some pins and a lanyard and participate in trading pins. You can buy some on eBay to get you started and then trade as you wander through. The kids loved it!