Its a 3.2 litre diesel, not a CLS 500 AMG, with all that implies if you're after ultimate performance and not worried about running costs.
That having been said I've been in the "ordinary" 500 CLS (not AMG!) and found it a generously sized car with amazing performance - not as clever on twisty stuff as smaller, lighter cars, but plenty of stop and go.
One of its problems on Irish roads is the sheer width of the thing - its not quite as wide as a supercar, but people with fast, wide cars always seem to forget the number of times they'll NEED their brakes because of the width of the vehicle.
One of the sleekest cars on the market, whatever the engine, and if you can live with the more pedestrian performance of the diesel you can portray yourself as a "concerned" rich git, as opposed to a totally over the top type.
Part of the sleekness is the length and even dedicated parking spaces can be a problem if access to them is tight, the legendary turning circle abilities of RWD Mercs notwithstanding - still, just learn to park further away and get fitter!
*Do* remember we are in a recession and if you regularly park this in areas where people are feeling more and more annoyed at the current economic situation, you may be visited with someone's unwelcome boot or key on the paintwork.
This happened to a more mundane e-class owner I know at the height of the tiger, and I expect that the feelings that motivated his cars dents may be running higher today - just be careful where you park.
Enjoy it while you can, before emissions control ruin motoring for everyone.
As for problems - its a Merc and things occasionally can go wrong and they're seldom cheap to fix - here are some links
http://www.evo.co.uk/carreviews/evolongtermtests/209307/mercedes_cls_350_cgi.html
http://en.wikipedia.org/wiki/Mercedes-Benz_CLS-Class
http://www.benzworld.org/forums/w219-cls-class/1230853-cls500-transmission-problems-anyone.html