This is my second BX. They are comfortable and rather good for long distance cruising, especially when their extremely cheap secondhand price is considered. The BX is an extremely boring car to drive, but is practical and great on motorways.
I thrashed my last one - with my foot down doing 100mph across Europe frequently and for hours/days on end. The head gasket went at about 120,000 - after twenty thousand miles of relative abuse - but I can't complain.
What is wrong with the car is the lack of room to work on it. You need special tools for many jobs on the hydraulic system (to reach into awkward spaces), and/or suffer cut and bruised hands trying to reach parts other beers cannot reach: under the body, behind the engine etc. If the hydraulic system goes at any point, there is a loss of power steering, brakes and suspension.
All too often, small faults occur in the hydraulic system which mean a lot of hassle, as well as bearing the huge cost of replacing all the LHM (hydraulic fluid). I have used Mineral Hydraulic fluid from farm suppliers for a long time with no apparent detriment (can't say I recommend this to anyone, it's up to you!) to the car or its performance - this costs about a pound fifty a litre instead of a tenner, but can be hard to find.
The engines in these cars are great - economical to run and not too dear for spares, but they are HELL to work on. Fixing a small fault in one place can mean a huge strip down just to access it. Had Citroen made this car with just a little more space around the engine block and sub frame so that it and the hydraulics were more accessible to work on, they would have produced an outstanding, excellent car.
Note for those wanting to sleep in the BX - if you unbolt the passenger seat and the back seat, and lay the back seat where the passenger seat should go - along the length of the car - you will find an excellent padded bed is made - (for one).
I run my engine now on alternative fuel 'Bio Diesel' made from waste cooking oils and a solvent (an enviro-friendly one, of course!) - it works fine, with an oil pre-heating heat exchanger (heated off the the cooling system). But I do have to switch my glow plugs on and turn her over once or twice in the winter months, and it gets a bit viscous at temperatures under minus three degrees - i.e. a heavy frost - at those temperatures it needs more additive, or Diesel mixed in with it.