A good way to kill time if you're utterly bored and have nothing to do, but overall, I think they're boring.
I guess they'd be okay as side games if you're into that kind of thing. Perhaps card games are a big thing in Japan and with younger players (like Pokemon and Yu Gi Oh!), but it's kinda odd to assume everyone wants to play it (the way they integrate it into the game and make a big deal of it).
I know a lot of RPG freaks, but I don't know a single one who enjoys the card games OR the Blitzball side game in the FF series.
The side games they DID love was in FF7 with the Chocobo Racing, but once you got the Ultimate Choco, there wasn't much point in doing it again.
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Dragon Warrior 4 had a casino with Video Poker.. I was so addicted to that, but you also got the best items in the game from there. Metal Babble shield was like 60,000 some chips
, but damn it was kickass.