I've had my 3520H for 2 years now, have 210-ish hours on it.
A majority of my time has been with the tractor nowhere near the "green RPMs", either driving around, posthole digging (I dare you to use a phd at full throttle), casual FEL work (forks and moving dirt / gravel), a small amount of tiller work (at PTO RPMs); for the most part I'm only at 2000+ RPMs when going up my hills or digging hard.
DPF: so far, I've only seen the light come on when the key is turned on. A couple times when driving up a steep hill at high RPMs, I've seen an exhaust which is probably soot being burned off, for about ten seconds.
I'm not worried at all about the potential for DPF clogging considering my use, and I'd prefer to keep the DPF intact regardless, because I like not smelling diesel except when refueling...
I am gonna jump on the Branson bandwagon as well. I have a 3015R I bought new in April, since I have put 42 hours on it in lots of different capacities and it continues to impress me.