Comparing Giant to some similar terminal $5 attacks is not a bad go.
Tr -> Dame Sylvia gives $4 and searches 4 cards for $3-6 to kill; however she dies if she hits a knight or is hit by another Knight.
Tr -> Jester gives $4 and searches 2 cards to gain value ($2-9, but rarely outside of $3 - 6), give out junk (usually coppers, but sometimes curses), or whiffs (hits an empty pile, Knight, or Bm card)
Tr -> Giant gives $6 and searches 2 cards and it MUST either destroy $3-6 in value xor give out a Curse. Dies if hit by another Giant.
So against Dame Sylvia (say from the Bm to make life easy) is clearly inferior if cards in the $3-6 window make up less than 1/4 of your deck, you will start whiffing with here while Giant will be tossing Curses and giving more cash. On the other hand against a really thin deck with good trashing Sylvia is far better at causing destruction, well worth $2.
Against Jester, Giant makes it easier to buy what you need and easier to give out stronger trash. Again Jester gets relatively better the more we are looking at gaining two $3-6/destroyer $3-6 vs giving 2 coppers/giving a curse.
This suggests that long term, Giant is a nicely balanced $5 attack.
However, in the short term, Giant can be a beast. $5 is enough to vault all the way to Plat off coppers. Getting Kc, Forge, or Prince early is huge and early curses are generally far more damaging than later curses. I can always buy a Giant after playing one for $5. This high variance can be extremely useful in the opening to hit one big and one tiny hand or in the late game to spike provinces in a busted deck.