Did you book the reservation in ARS or USD?

I didn’t know until now.

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One small MC purchase on 2 Jan.

Posted at $177.06. Credit 4 days later. After refund, $310.86.

This appears to be working smoothly now. Paid with a UK issued Mastercard last week to test it out. Transaction posted at the official rate and a refund appeared 4 days later making the actual exchange rate approx 372 pesos/£ compared to Western Union's 389 (after fee) and a blue rate from a cueva of approx 420. Still worth paying for everything in cash if you can

Second small attempt - 5 days for the credit (Chase MC).

I have now tested a US based MC in addition to my Visa. The MC was used for a ARS 9500 purchase. Initial charge was $53.22. Credit 3 days later was $22.42. Net charge was $30.80 for an effective rate of ARS 308 / USD. So my Visa is getting a slightly better rate. The visa continues to post at a rate of about ARS 312 / USD.

Curious about this as well as I'm starting to plan this year's annual trek to Argentina. Will be so glad to not have to carry bags of cash.
Don't they have to agree to settle the bill in ARS? If so, the big trip-up would be if one inadvertently selects the horrible Dynamic Currency Conversion (DCC) option on the credit card processing machine and then it gets charged to the card in USD at a conversion rate even worse than the official rate?

Thanks everyone for the data points. Looks like I will use Visa or Mastercard as well as send some money to myself via Western Union. I guess I can pick up the Arg pesos at any Western Union in B.A.?? Or are their some offices that are better (less crowded or have enough pesos for $500 US sent)? I would send more $$, but heard if you send too much, issues can develop. Maybe send some now and pick up in 8 days when I arrive and then decide if another $500 -- or just take the Visa / Mastercard conversion rate. I have no experience with such transactions - last time in Argentina was about 5 years ago.

Again,, thanks.

If it's only $500, I would suggest just finding a local exchange place versus having to search out the Western Union office. I'm not sure you'd end up coming out materially ahead with the Western Union rate.

No, not all will (or were unwilling to give me). Take a look up for the one I'd gone to; you'll have to wait in line a while, but I'd done a total of $750USD without issue.

OSKI You are probably right. My concern was the new credit card rate (a bit lower than Blue Rate from what i have read) would not be updated or would be changed to fall back to Official Rate. I will likely try some $500 - $700 (as suggested) amount to be a 2nd method of getting a decent US $ to Arg Peso rate. And taking some fresh $100 US bills with me also. kennycrudup info was helpful as well. Will try to remember to post data points late next week after arrival.

$500 USD is a Giant wad of ARS bring something to carry it in

As another poster above pointed out, if you pay cash for your hotel, you're subject to the 20% tax. So, even if the Visa/MC rate is a bit less than the full cash/Western Union exchange rate, you're probably still well ahead using the credit card. And, you don't have to carry a bag of cash, which is what I literally saw last year at the Hyatt. It was kind of comical, but also a bit cumbersome to walk around with that much cash.

If you need an exchange place, your hotel can usually refer you to someplace nearby that is reputable.

Especially if they pay you in 100s.....

Is this working for Visa or Amex yet?

Visa, yes. Amex, no.