It's impossible to use on your first turn since you can't load up the Graveyard before using it until your next turn, and by then it's likely too late to make any kind of impact
The opportunity cost to include them in your deck is so low that it often makes no sense not to play them, outside of a few hyper-specific cases
Look up recent sales for the same card, paying attention to grading and edition, and compare them to yours to gauge its value
Its a better set of stats for Crewing, and the cost reduction ability plays nicely with cantrip artifacts, Puzzleknots, Skysovereign, Metalwork Colossus, and more
The Yanmega ex deck attempts to do exactly this, utilizing Dialga ex to quickly charge its attack and start firing off Air Slashes at the opponent's team
It also represents a lot of damage, as youre adding +2/+2 and haste to a creature, besides the extra combat step