I'll give my $.02.
I have owned 2 - 315CL's, a 321CLCR and a 325DL. I put 4k hours on the first 315 and 1K hours on the second, I never turned a bolt other than wear and service items. The 315's are great machines in my experience.
The 321 had 3500 hrs when I sold it. No problems with that machine other than breaking the dog bones on the progressive link hydraulic thumb. The dealer came out a few times to reset the pressures on the thumb and after breaking two sets of Cat factory dog bones, the Dealer fab'd some beefier ones up in their shop and we never had another problem.
The 325 I put 3K hours on when I traded it in. It was a very good machine, no problems with it and probably my most favorite machine to run out of the hoes I have owned. I demo'd a PC300 and the 325 on the same site before I bought that machine. I liked the 300, although to me, it seemed slightly weaker than the 325 - the kicker was the 300 was $40K more.:beatsme It was an easy decision for me.
That's my experience with Cat trackhoes, worth price charged.