I use legendaries but very rarely for my in game team (exceptions being Mew in Red and Victini my first play through of white). But post game I'll deffinately use them. For competition I'll use the lower BST legendaries if they fit my team and then for training weaker Pokemon I'll use Ubers to crush the 10th level of Black tower. I used Mewtwo to sweep the elite 4 and champion to get the 1 and only medal didn't even need to use a healing item.
Please check out my blog/guide to competitive pokemon at pkmntrainercory.blogspot.com
In the first few generations I used Suicine, Raikou, Zapdos, and Articuno some but anymore I just try to get a decent one, then box it. If I decide I want to use it for battling later, I can use it then, but they pretty well just hang in my box.
I always used them in Red/Blue and Gold/Silver, but haven't had one on my team ever since Gen III. I don't know why, I guess it feels a little bit cheap to use an overpowered legendary. Even when I was more or less supposed to use it against N, I said: screw that, I'm going to end this with the pokémon that have been with me from the start. And I did. And it was awesome.
And I guess it also caresses my ego that the god of all pokémon is stored inside my computer and can only come out when I give my permission.