This blackjack variant has been around for a while but has never really caught on--except for in the Pacific Northwest. I've been told that it's not the game itself that has made it popular; rather, it's the Match the Dealer side bets.
Maybe someone can confirm my speculation on this: WA has always had a lot of small casinos where only table games and poker are allowed--no slots. So those casinos wanted to give people ways to make longshot bets that pay off big in the remote chance that they hit. That's why every table game in WA has some kind of progressive side bet--and of course, all those bets are high house percentage.
So I play in OR and WA and I watch people bet more--sometimes MUCH more--on the Match bets than they do on the primary bet. And good God, do they play badly. There's a specific Basic Strategy for Spanish 21 that is drastically different than that used for regular blackjack. It's based on the fact that all the 10s are removed from the deck. For instance, you always hit a hard 12, almost always hit a hard 13, and most of the time, hit a hard 14. The Wizard of Odds has the Basic Strategy on his website. Players whom I've observed almost all don't hit enough.
The upshot is that people get KILLED at this game. The Match bets carry a house advantage of almost 6 percent at many casinos (the paytables are much worse than those on the Wiz's website), and the very poor play of the hands exhibited by almost all players costs them at least 4% on their main bets. The game is a complete money sink.
The ironic part is that if you know the proper Basic Strategy, this game can be better than almost any other blackjack game out there. Plus, a LOT of time per deal is spent resolving those stupid Match bets, so the game moves very slowly compared to regular BJ--which, as you know, means you lose less.
I'm curious to know if anyone has played this game seriously. I'm given to understand that it's countable, but not in the same way regular BJ is. Most of the pit critters appear to think that SP21 can't be beaten, so anecdotally, APs who play it get little or no heat. Katarina Walker (deceased) published what appears to be the only definitive text on this game. It's widespread throughout WA and OR, so there may still be opportunities. I'd appreciate it if anyone has direct experience or knowledge about the playability of SP21 in those states.