Well, my machine showed up at the dealer today. Will be waiting until Friday for my trailer to show up, so I sadly didn't get to take it home today as I'd like to, but I did play with it for about 2 hours around the dealership today.
My first impression is that it is by NO means underpowered. I've seen plenty of complaints on the forums concerning the only 7hp rated difference between the T320 and T870, and was starting to get a bit paranoid after ordering. I'm happy to report that there is a significant difference (in my opinion- and I have used both a 300 and 320 previously) in the amount of pep that it has. Track speed seems to be a bit faster, overall movements in hydraulics are faster and from what I can tell the overall power of the hydraulics are up. Not sure where it comes from- might be the extra 300psi of hydraulic pressure, better pump/engine matching, or maybe just better geometry of the arm and bucket mechanisms but it really does feel better.
The interior is pretty much as nice as what the claim on the M-series. Between the air ride seat and the roller suspension it is just stupid smooth for a skid/track loader of any kind- honestly feels more like a somewhat stiff truck than a piece of equipment. Engine noise both inside and out is a good deal quieter.
My concerns about the serviceability were pleasantly met. Compared to the K series stuff, it looks to be a pleasure to work on. Visibility inside the cab is much better, and the bells and whistles sure do make a day's work inside seem a lot more attractive.
There are a few small drawbacks compared to some of the other machine selections. First off, its just too damn wide to fit between the wheel wells on a normal equipment trailer. Had to order a 10T tandem/dual wheel gooseneck for our F450 to fit both the size and weight of the machine. We will be using it on 35 acres of fairly open land, but I wouldn't expect it to fit anywhere exceptionally tight and still have room to work without lots of four-letter words. Secondly, I will have to say it will take a bit of getting used to with the longer "tailswing" (for a lack of better term) with the longer machine and cab forward. There may only be a minimal actual difference in how much the ass end swings out, but it certainly feels like more because of the positioning of the cab.
If your looking for something to replace a small dozer (somewhat literally) and still work great as a CTL in open places, I think anyone would be very happy, and comfy, in the 870's. Just don't expect it to fit in small spaces, and you'll have to overlook the sticker shock.
I'll post up some pics when I get the machine home.