As a play, theyre decent, especially if youre playing at a higher power level and need to not rely on an opponent (misdirect) or cant have summoning sickness get in the way of your creature (needing haste)
(9/15/2013) You choose how to order cards returned to your library after scrying no matter where you put them
Gyarados is a beautiful card with a powerful attack, but it takes quite a bit of set-up, which mostly revolves around keeping Magikarp alive long enough to get Gyarados into play
I'm also leaving out Legendary Dragons, so remember that these are what I consider to be the best of the best of the Dragons you can only run in the 99 of your EDH deck
All three have the common theme that you'll get something valuable from your foes, but you cannot plan around it, since they control the effect
The most common mistake is filling the plan slots first and treating the functional categories as optional