I think for Disneyland, it's a rather excellent value. Highly comparable prices to many other steak houses that serve great meals. I had either the porterhouse or ribeye last time and it was excellent. Plus, you have a discount for AP holders making it an even better value.
My candid assessment is one of disappointment quite frankly. The service is excellent and the ambiance is nice; however, I have been on multiple occasions and found the steaks to be uniformly overcooked from the ordered medium rare. Notably, the last time I was there with five others in my party, and we all ordered our steaks medium rare. Not one of them was very pink at all, and they were dry. The staff were very polite and immediately willing to take them back, but we were well into dinner and on a time schedule and so didn't send them back.
For the money, I strongly recommend Mr. Stoxx on Katela. It's not just a steakhouse, but they have outstanding steaks. And of course, there's Ruth's Chris on Harbor, which is amazing but pricier than Steakhouse 55 and Mr. Stoxx.
For breakfast I argue this is the best place to eat in the entire resort. As for dinner, it is very good, but not great. It won't blow you away for the price, but you will feel like you got your money's worth in quality. It's a much better breakfast place than dinner. I am basing this on the price for breakfaast and what you get. Price is comparable to carnation cafe for breakfast, but MUCH better!
Just ate there on Valentines Day (my girlfriend surprised me) and I had the best prime rib ever. The potatoes au gratin were so so, and my girlfriend loved the salmon. Very good service and very good food.
PS - Get the "55 Butter" on top of your steak. Trust me.