i don't think mine's been hugely unreliable (in the sense that it has left me stranded - although the gearbox did seize whilst i was out and the battery also failed on me away from home..) but - a lot of parts seem to have a disappointing service life.
there are also a lot of people who (apparently) have faultless cars but i'll bet that, if they try hard enough, they'll find that the same components that wear in everyone else's cars are wearing in theirs
i've only had mine a year and don't know anywhere near as much as some of the aficionados on here but i'd list the following as problems that you should budget (depending upon the individual car) on having to replace, sooner or later:
- top mounts (poor design)
- engine mounts (too soft)
- exhaust and hangers (corrosion and, again, rubbers are too soft)
- rear dampers (they leak)
only a few thoughts - as said, lots of other things to look for listed in the guides (sorry, don't know how to link to them)