I took the 25+ hour train Retiro-Tucumán last May 2011 to attend the 80th birthday of my second cousin there. Went with my girlfriend, we're in our 50's and we 'splurged' on a camarote (private compartment with bunkbeds & a sink). Although the price on-line was ~750 pesos (1-way), I went to the ticket window inside Retiro train station (at the very end, toward Libertador) a few days prior and got an unpublished promo price of ~540 pesos, for TWO people, including breakfast. In the dining car we spent another ~75 pesos each for lunch & dinner, which were decent, not bad but not great. The green train departs ~9:30am every 2-3 days (check the schedule in-person).
The train rarely arrives on time (ours was 2 hours late), but we weren't in any hurry. Was fun for the first 16-20 hours, we were able to sleep fine during the night, but those last few hours really dragged. They've got electricity in the camarote so you can recharge your PC/phone/etc. Yes the plastic window protection was a bit foggy & scratched in places but in truth the view was fine. It's fun to wander around and admire the view from different perspectives, plenty of porters & train employees so we didn't feel unsafe. The shared bathrooms on each car got a bit raunchy after 16 hours, and yes, their were occasional roaches (small, discrete & rather well-behaved). The camarote might feel tight for more than 2 people, and the kids might get really stircrazy depending on their ages. Yes you see lots of country but there were no "on-off" privileges during the many brief stops, hence not much interaction.
Would I do it again? Probably not, but it was a fun 1-time adventure. My relatives & friends here thought I/we were crazy due to the duration. I've taken overnight microbuses as well and they're definitely easier, faster, but more expensive (especially the full 'camas' with 180-degree seats). You might also check for airfare promos which approach the busfare, especially with the little airline 'Andean' that only goes to Salta. (BTW I've routinely paid the 'Nacional' price buying and checking-in online in/out of Aeroparque, using my USA passport number-1 digit as DNI, but it's a risk so be prepared to pay the difference).
Suerte!