I have owned this car for almost 7 years and have had very little trouble with it - possibly because of the low mileage and regular servicing etc.
The only real mechanical problem in all that time has been the CV joints that knock when turning tightly and accelerating at the same time - apparently this is quite common (as are other steering/knocking problems!). The CV joints will need to be replaced next service.
I read about the infamous sunroof seals that all seem to fail (UV from the sun perishes the rubber) and sure enough, last year it started dripping in - after filling up the remote locking receiver. That has stopped the sensor from working (except from inside the car!) so I'm back to using a key (!) but worse than that... it seems to unlock itself in the rain every now and again! I will have to sort that out soon but apparently it is about £40 from Citroen for the seal (and scrap ones will be useless as all Saxos have the same fault!) plus I will probably need to replace the sensor and re-program the remote.
Other than the faults above, there is very little bad I have to say about the car. The brakes are pretty weak (even when brand new) and it seems very easy to break into (3 times in 2 years when I lived in central London). Also, it isn't particularly comfortable - it's pretty bouncy - especially with passengers or a full load in the car.
Otherwise, it's great - I never planned to keep it this long but I am so happy with it, it would cost at least £5,000 to get a better car. The handling in particular is excellent and it can be thrown around corners without fear of body roll or under steer. The performance is pretty good without being high performance - you'll want a VTR or VTS for that. It has more than enough power for general driving and accelerates up hill etc. For a design over 10 years old, it also looks pretty good - the skirts and bumpers etc from the VTR range give a vast improvement over the standard body style but it doesn't have the spoiler so doesn't look too much like a chav mobile!
Running costs are good - driven sensibly, fuel consumption is pretty low - I don't know mpg figures but about £10-12 should get you about 100 miles. It is in the relatively low insurance group 5 and tax isn't too bad either.