I've never been on a guided trip, but I did use Aymara in February 2010, just for mules, a tent site at Plaza de Mulas, toilet etc, plus a night at Confluencia on the way in with dinner and breakfast. It was just me and my girlfriend. They were fine, very easy and helpful in booking the mules (in their office in town). Only glitch was they didn't have enough tents at Confluencia so I carried our tent in to there, used it, then put it on the right mules the next morning. This was a non-standard thing to do, thus providing the opportunity for things to get messed up, but the Aymara staff at Confluencia made sure this happened correctly, and their breakfast and dinner were fine.
I did not eat at Plaza de Mulas or use their services in any other way. The toilet was a bit dirty, but from what I've seen most of the companies are the same. Climbers not shitting all over the floor would help a lot. Back at Puenta del Inca the Aymara staff handled our mule bags in and out with no hitches, did what they said they would do, roughly when they said they would do it. I had no complaints and would use them again.
I'm not surprised that any company there would have kinks and glitches. There are a number of companies trying to service a market that is often looking for the lowest price, but in an area, and for a task, that is often difficult and sometimes dangerous. I saw some good guides, I saw some bad guides - from every company. Some are too quick to turn around, some not quick enough. I saw people who were obviously trying to skimp on their guided trip when really they needed a bit more assistance - and time - than what they'd wanted to pay for.
Guiding on Aconcagua is a very variable and imperfect situation. It can be great, or terrible. Anyone using these local services, or handing off the job of running a trip to a mountain like this to any company, or individual, needs to do their research and accept a level of uncertainty. You could have a good or bad experience with any one of Aymara, Inka, Grajales etc etc. The more you can look after yourself and reduce the reliance on these companies, the better off you will be.