I've always used a very healthy dose of "man maths" when it comes to the switch from one car to another - but as my wife says "at least thats your only expensive vice"
I just moved from a 4 year old M135i - which i'd taken to the dealer for a vehicle inspection and brake fluid change.
The price to re-extend the warranty, add a new service pack, and do the MOT - just seemed like money thrown at an oldish car. Had a quick test drive of an X3 M40i - realised it was fantastic - so gave the sales guy some numbers to aim for - told him I could move immediately if he could do a deal - and waited for his call.
Month end rapidly approaching - and he wanted a sale - so gave me a great deal.
Its more money than I was paying before on PCP - but not that much more - and the man maths closed that gap for me
Definitely echo the above - if you can afford it - will enjoy it - and have pushed the sales people really hard to get a good price (but not so hard that they don't want your business) - then go for it.