I just went to Santiago from Mendoza last week and it wasn't a hassle but I was nervous about it. My visa was expired and didn't know if I'd be able to pay the fine at the immigration centre up in the mountains. Turns out the guy just looked at my passport then asked if I had a "paper". I said no and he just shook his head and then stamped the little form you have to fill out before reaching immigration (on the Bus). No fine. Then I was into Chile.
Coming back, there was a long wait at immigration but I was one of the fastest through. The chile guy stamped the paper and when I went over to Argentina immigration they just took the paper, opened up my passport and stamped it. They didn't look through it at all.
Also, at both sides of immigration they didn't "swipe" my passport or enter anything into a computer. I also wasn't charged any money such as the 5 pesos I had to pay when I came back from Uruguay.
Would I recommend what I did? Not really, since there was a highway check by the national guard when they stopped the Bus outside Mendoza. They came on and checked my passport. I got through because I was nice about it and when they asked if I speak spanish, I said no. They just sort of smiled at each other and handed my passport back and gave me the thumbs up. My guess is they didn't want to hassle me. I was expecting to be arrested and taken to an immigration office and was half out of my seat the whole time expecting to be led away.