After having played with Charm a few times now, I like it, but I find I'm using it for a Silver +Buy more often then I'd like. You buy it thinking it would great to buy two $5's in one turn, but that's not so trivial when you need to get $5 without using the $2 from Charm. I think it's best when there's multiple $4's or multiple $3's you want, say a $4 village and a smithy type card, or a Village/Menagerie/Oracle or something. If the other $5's on the board are really important and the $3's and $4's are meh, I might skip Charm and just buy the $5 I want earlier.
I did have one game where I was able to use Charm similar to a HoP Megaturn. Not exactly the same, but in a deck that can draw itself, play two or three charms, and then has enough money to buy 3 or 4 cards (not a ton of money, just $12 or $13 or something), suddenly you're buying 4 cards and gaining another 12. This is where Charm is at it's most fun. If you're buying it and only gaining one extra card every 2 or 3 or 4 turns, it's not very exciting.
EDIT: This is against the rules so don't do this! See below