I went through it a couple of days ago and had to wait only 5 minutes, the new hall seems quite efficient.

There are recent wait reports of over an hour. The only thing I know to expect is that I never know what to expect.

What time was it when you went through immigration/customs? My last EZE landing was in July and immigration was reasonable thanks to the new e-gates (for Argentine passport only), but I have heard pretty bad experiences over the last few months, particularly in the "extranjeros" lane.

We landed at EZE on 9 November around 10:15 am and the immigration wait was between 40-50 minutes. The line actually moves pretty quickly, but one or two other flights landed around the same time. We were flying at the front of the plane, so had we not taken our time deplaning and a restroom break, we probably would have been in line ahead of other people (our connecting flight out of AEP was delayed by 2 hours so we decided not to rush). We did not have any checked luggage. One thing I noticed about the immigration hall and basically the entire airport was that the AC was not powerful enough and felt like it was non-existent, so that felt quite miserable.

One month ago it took me 30 minutes to leave the plane ( BA ex LHR ) , go though immigration ( I ama an Arg resident) , pick up my bags and clear customs - the best time in about 5 years.

Last week it took my sister one hour and 20 minutes , arriving on the same flight at the same time

Yep - it really is pot luck. Same flight for us the other week (BA245) - landed early at 0840; sat on the tarmac waiting for the gate to become free for half an hour during which a couple of other heavies from Europe taxiied past us to their gates. Disembarked at 0910; Cleared immigration and customs at 1100. That's the worst wait I've ever had

Are there particular days/times that are worse than others? I'm going for the first time in two weeks - arriving Friday around 11am, leaving Monday at 9pm. Any reports on what to expect?

The only thing you can expect is that you never know what to expect.
Jan 2 at approx. 5pm, there was almost no immigration wait for arrivals, and baggage was relatively efficient. No customs lines to speak of.
Jan 8 at approx. 7pm, there was a very short wait for security, and then maybe 10 to 15 minutes for exit migraciones. I was rather shocked. Given the sea of lost humanity waiting in various lines in check-in counters, I suspect those lines became a lot worse shortly thereafter.

I personally avoid those arrival/departure times but only because I'm fortunate enough to be able to do so.

As said it can be a bit of a lottery however overall it does seem to have got a lot better than it used to be when over one hour was normal. I wouldn't worry about it, just go with the flow.

I've had it so hit and miss at any hour that I just know to expect that I never know what to expect, as I've already said. I've arrived EZE (for departures) during very slow times when there's only 1 WTMD open, and very few immigration officers, so the lines back up substantially. I've been there during peak hours and things flow relatively smoothly.

As a point of reference, how frequently do you arrive EZE on an international flight?

Wait times EZE
 

Not sure I'm posting correctly, but I have two options for flying through EZE (on way to Ushuaia): Delta (arriving 10 am, which gives us 3 hours to clear immigration & customs and re-check in) and American (arriving 9 am, with a 4 hour layover). Would prefer to do Delta but travel agent is advising American to give enough time. We're traveling business. Any advice appreciated.