I wrote about the problems with the hot end before. Though I did get the hot end to work, it only worked at slow speeds (25 mm/s) and the extruder motor was visibly loaded more that what should be necessary. The extruder's hobbed bolt also slipped regularly leaving holes or weak parts in the printed objects.
The friction of getting the filament through the hot end (0.5 mm nozzle) was just too high. I decided to try another hot end, the J-Head, and this has been working fine for 2 weeks now. I bought the hot end here: http://reprap.me/
As the top part of the J-Head is bolted directly to a part made of PLA, I wanted to make sure the cooling was taken care of properly: a separate fan is blowing air through the holes of the J-Head to make sure the top part will not melt the PLA.
To cool down the printed PLA quickly I currently use another, stationary, fan. This caused a problem with reliable flow of plastic because the heater block was now being cooled too much. Too much cooling was prevented by insulating the heater block (not visible in the picture about yet) with some ceramic tape. I will make another solution for the PLA cooling later, but for now the quality (and speed) of the prints is greatly improved.
The Stanford Bunny from thingiverse:
0.15 mm layers, object about 6 cm high.