The older Vauxhall Vectra B's are still in regular use by the police here and seem to be holding up well, despite being fitted with bullet proof glass and armour plating. So they're pretty rugged machines
(and I'm not just saying that since I had a Vectra B and loved it... ok maybe I am). I'm not sure if they keep them because it's cheaper to fix them than replace them all though. The sheer number of them is staggering. I think when they finally die they're primarily replaced by Skodas. The PSNI seems to have gone with the Ford Focus for Panda patrol though.
http://www.psni.police.uk/psni_fleet-2.pdf
That shows what's in use here. Despite the number of Mitsi Shoguns on that list, I haven't seen one in years. Also, it's kind of weird that a quick google returned that list, but 15 or so years ago if you'd compiled it yourself, you'd have been charged with "collecting information likely to be of use to terrorists".