My experience with an UM2 so far:
I bought my UM2 in May 2015 from the german distributor ELV, as that was the only one i recognized (I wasn't aware of the special status of iGo3D back then).
The printer was working 'ok'. After some days, I discovered a small crack in the top part of the print head, but wasn't sure if that one existed before. Actually, the printer didn't look really new when unboxing, but I trusted ELV and didn't think about it any further. I've just been happy to be the owner of such a cool device, and didn't want to return it.
My print objects where primarily cases for electronic devices, so I needed to print in ABS. To simplify the process, I bought a special plate (Dauerdruckplatte). That one was working very nicely for ABS, but only with a heated bed temperature of 110°C, and only for very small objects. When printing larger objects, they didn't stick to the build plate evenly, no matter how often I calibrated the build plate. Same effect using ABS/Acetone on glas. So after a while, I got frustrated and lost interest.
Later in 2015, i stumbled upon a forum entry, where somebody mentioned similar issues, and found that his glass plate was uneven. While that sounded unlikely, I checked my glas plate, and realized that it was indeed not very even. So, I ordered a new plate, and it made a big difference.
For a while, my UM2 has been creating successful prints, while the crack on the print head slowly grew. When Ultimaker announced the Extruder Upgrade kit, I ordered it. But didn't have the time to install it, until recently, when the original print head fell apart. I didn't worry too much, as the upgrade included a brand new head.
After installing the upgrade kit, it indeed created better results. At first, I've been unable to properly print ABS, as the new UM2+ firmware was limited to 100°C, and ABS didn't stick to the plate anymore. But iGo3D support was incredibly helpful, and after installing the very latest firmware, I've been able to create perfect ABS prints again, on a 110°C hot plate, without warping.
Imho the new feeder also makes a huge difference, especially in regards to retraction. Only problem I got now: After two perfect prints, that looked better than ever before, the Y-axis stopped moving...
I unmounted the Y-axis stepper motor, and confirmed that the axis alone moves nicely. Indeed the Y-axis stepper hardly moved, even if the printer was turned off. In addition, when it moved, it looked like the stepper axis was bent, but it turned out that the hole in the timing pulley was causing that behaviour, as it was not really centered.
So now I ordered a new Y-axis stepper, and a timing pulley replacement, both from iGo3D. Actually, those will be the last spare parts I order, as I'm pretty much fed up with this. Also I start thinking that I might have been better off with one of the chinese clones, considering the abysmal quality of my original Ultimaker2. Not sure if others share a similar experience?