Test driving the ZX3 with manual transmission sold me on this car's fun to drive qualities. However, having owned many cars (including 3 Fords) I can honestly say this car was the biggest lemon I ever had the displeasure of owning. Over the course of 84k mile ownership I replaced the AC system, valve cover gasket, ignition mechanism, EGR valve. The car was fun to drive and got great gas mileage but left me stranded more times than every other car I have owned combined. Amazingly, mechanics and tow truck drivers could usually guess what my problem was because they had seen so many Focus with the same issues. It feels like the sun has shone brighter every day since I got rid of it!
Fun to drive, great gas mileage, lots of headroom, low purchase price.
Engine, transmission, brakes, just about everything else. Based my experience and on talking with other ownership victims I would guess a combination of ineffective design and poor quality control.