I just flew TAM from GRU - POA two days ago and am actually at POA right now awaiting the flight back. TAM allows online check-in up to 72 hours in advance! Yep, 72 hours. I highly recommend it if you aren't checking luggage.

Can't comment on lounges since I only have EXP on AA and no status on * Alliance.

Connections are generally ok and all domestic flights have a drink and snack service.

Like the OP said, JJ flights often have delays but connections seem to work. If you are going somewhere with very limited connections, add a bit more time if you can

Not a ton of legroom in coach, but no worse than anyone else I've been on.