On my Emerald version, I "wasted" it on Rayquaza. That Level 70 son-of-a-***** kept using Extremespeed, and i was getting pissed off. Now on my FireRed version, I still have it and I'm keeping for something else. I was able to catch Mewtwo with a standard Pokeball on the first throw, though (after knocking it down to one HP with False Swipe, paralyzing it, putting it to sleep, THEN wasting dozens of Greatballs and Ultraballs on it). -___-
Edit: After reading alot of the earlier comments about catching the roaming Pokemon, I have this to say: catching Latias with an Ultraball was probably my single most fulfilling Pokemon experience. I learned that if you get a hit in before it runs, the damage stays whenever you encounter it again; and that you can prevent it from running and give it a status effect by leading with a Pokemon that has an ability to prevent fleeing. An example would be a Wynaut/Wobbuffet with the "Shadow Tag" ability.