Vacationed in Buenos Aires in August 2007 and stayed at the Marriott Plaza for six nights. Hotel location was excellent -- right at the end of the busy pedestrian shopping district and within walking distance to the subway (which is cheap and relatively convenient).
Had a junior suite, which was spacious, comfortable and clean. Bathroom was very large with separate rooms for the sinks and shower/toilet, and there was plenty of storage space. Only downside of our particular room was the view, which was of the interior of the complex and not at all attractive.
With our junior suite we had access to the Executive Lounge, which was a nice perk. They had snacks and beverages throughout the day, and the service was excellent. The breakfast (also included in our reservation) was adequate though chaotic at times, as it was very crowded most mornings. They had the same fare every day, so after six days it got old, but it was convenient. It's a busy hotel, so the small lobby is usually chaotic, but we had no service problems.
There are nicer parts of Buenos Aires (Ricoleta is within walking distance and there are nice hotels in that district) but we have no regrets about staying at the Marriott. Would definitely stay there on a return trip to BA.