On my first time playing Heart Gold I used a Synchronize user to catch a Jolly shiny Gyarados who had 31 IVs in Speed and decent IVs on it's other stats (I think...)
Then I transfered my Gyarados to Platinum to EV train him. I used him in the Battle Frontier and some Wi-fi battles 8D
In my other playthroughs I got 2 Adamant shiny Gyarados. They have been sitting in the boxes forever, waiting for the day I'm not too lazy to EV train them.
If I ever play HG/SS for the 4th time I plan to actually use the shiny Gyarados in my in-game team.