Sorry, I just saw that post as reading something like: "What's wrong with them? Other than the fact they can't keep them running." You have to understand that from my experience, since they went away from the 200 series, they have gone down hill dramatically. Now the 200 series wasn't a fast machine by any means, but I have seen very few issues with them reliability wise. We had a 235C that would drop pump drives every 1000 hours just like clockwork, but that was the biggest problem we seemed to have.
Now, admittedly, the newer series have improved a great deal, but they just don't seem to stand up mechanically in the long haul and still haven't shown me anything to prove they are a contender in the performance category. Like I said, structurally, they are strong, but the controllability aspect falls right off when the hours add up. It's like the become uncoordinated hydraulically, and nothing seems to bring them back short of a complete rebuild/replacement of the hyd end of things. My experience where we were comparing fuel consumption was with a brand new 320C and an older Deere 200C. We traded operators between the two to make sure it wasn't a one sided contest, and the Deere burned 2/3 of the fuel the Cat was. The Deere was also making 4 complete cycles vs the Cat's 3 complete cycles in the same amount of time, working side by side in the same conditions.
I haven't spent much time with the D series, but that is mainly due to the fact that nobody here really wants to buy one. I have seen one D series Cat sitting around, and they were working their older Hitachi instead. That tells me something, but I don't know the reasoning behind it.
I know our other mine bought a 365C a couple years ago, and I have never heard anything good about it from operators, or mechanics. Last I heard, they were wishing they had bought a Hitachi instead, but went for the Cat as at the time we were having parts availability issues and long lead times to get parts for the Hitachi machines. That seems to have straightened out from what I have heard.
So availability aside, my biggest issue with them has been speed, coupled with a marked loss of controllability as they age.
Undercarriage wise, they are hard to beat. Structurally, they are right near the top or right at the top. Of course, Cat makes a good engine, but it seems a bit less efficient/economical that the competition. However, the hydraulic system and controls left a lot to be desired.