In the games I never use them, finally getting around to beating N in the final battle of the first half of Black 1, and I switched out the legendary right away for my Haxoraus.

Competitively...well the person who got me really hard core into Pokemon always looked down on using Legendarys. That being said even though he would have a random nature and is among the weakest of the legendarys I am really tempted to put Giratina on my battle team because the poor guy gets no respect and I think he's awesome. Though as I said I know he is one of the weakest legendarys.