For some reason the left passenger window stops working.
The engine would heat up a lot until I replaced the thermostat with one that was set up at a lower temperature, the original one would open up at a very high temperature. (not really a fault since I live in Los Angeles)
The head gasket likes to leak oil after a couple of years, as all German cars do.
For a period of about 2 months, some fuses kept going out. One of those fuses went out twice.
The throttle cable rubber broke (keeps the throttle cable in place, if rubber brakes you cannot accelerate). The rubber was 16 years old however, so it's no big surprise. However, I think it was rather stupid of Mercedes engineers to secure this cable to the engine with rubber, they could have used metal. It was slightly embarrassing though to get home at 15 mph.
Other than that, maybe several other minor issues, but nothing major though.
Overall this is a great car. Reliability has been pretty good; however, most things have been replaced proactively.
The radiator was changed, the water pump, the smog pump (CA car), et cetera... were all changed before they "had a chance" to break down.
The car handles well, fuel economy is pretty good, a 5 speed automatic would have made it better though.
The car's interior is better than most of today's car such as Nissan, Toyota, Honda, and other such cars. Luxury cars probably have a slight advantage although I've heard that many entry level luxury cars (3-series, c-class, IS) are full of cheap plastic. The 190 is definitely not like that, it has a quality feel to it.
However, the interior (dashboard) does develop some rattles at high rpm, usually at around 5 thousand rpm, and also when I go about 80mph it sometimes rattles for a bit, but if I slow down or speed up it goes away. If you look on this website, you will find tens or maybe even hundreds of people complaining about rattles in their new cars, this car is 17 years old, so I think it deserves a break about this, and even at their worst, the rattles are minimal.
The 2.6 I have is slightly underpowered sometimes, especially when accelerating from a stand point on a hill. I have heard manuals have the same issue, although they probably fare a bit better than an automatic. I have found out that the best way to accelerate sometimes is by giving it full throttle, not kick down. Full throttle is about 3 quarters of the way down. Still, it's underpowered many times, which is my biggest complaint.
AC in these cars is pretty good, I still have the original AC system in it, not the updated R140 or whatever it's called.
If buying a car like this, records are extremely important (actually they're important for any car). Check the car thoroughly and take it to a mechanic. They're pretty reliable but parts are not cheap, so be expectant of that...
To sum things up, this car is what Mercedes should be making now, the successor C-class and all the C-classes after that suffer from bad reliability (along with most of today's Mercs).
They should really look back at their best cars and see what made them so... and then put all that into their modern cars, because I definitely would not buy a new C-Class, I just don't trust Mercedes anymore. I have heard they are getting back up though, so maybe in a few years they'll be at the top of their game like they used to be.