Some cars, like the Omega, depreciated very heavily in their early years. This is in part due to the fact that the in many cases fleet buyers sells them after a year or so and this causes over-supply, and in part because the fleet buyers never actually pay retail price – they pay a heavily discounted price in the first place and sell the cars accordingly.
This is true for any non-prestige large saloon, including all Japanese execs apart Lexus (i.e. Nissan QX, Honda legend, Mazda Xedos and 6), as well as the Volkswagen Phaeton. With these cars, unless you are particularly rich or own a small oil filed in the North Sea, it makes perfect sense to buy them second-hand and let someone else - usually a large company - shoulder the depreciation.
However – some cars do not depreciate that badly, and other do not depreciate at all… the latter include the Mini and the Merc E-Series 320 CDI, in both cases you are likely to pay for a nearly-new car almost the same as for a new one… you would typically get some options for free, but that’s about it.
Cars with low depreciation are cars that were NOT favoured by fleets, such as some small Japanese, French, and German cars. In this case it can make sense to buy a new one – if you get the right deal.
Personally I bought my two Omegas as nearly new (the first was 10 months old, the second 5 months), in both cases for around £10k less than the list price. And if you consider the fact that even the full retail price was very good value compared to the German makes, then buying a nearly-new Omega is a deal you can afford to miss… only problem is, they don’t make ‘em anymore.