Is it worth buying a BMW M5 or is any 5 series just as good

If you've got the bucks to pay the premium, go for it. The difference between a 5 series & an M5 is definitely discernible. You will get goodies on the M that you can not get even on a Sport . Better resale , too.

Make no mistake - BMW is M cars are superb machines, however, there is nowhere in this country where one can (legally) drive an M5 to its full potential. Maintenance costs after the warranty has expired can be stratospheric. l traded my E39 M5 for an 2007 E60 M5 for this very reason. My wife is 2008 535i with twin turbos has more than enough power for US driving.

None of them are good. The BMW M5 and 5 series failed the side crash tests.

l have driven the 5 series, not the M5. And l have driven them many times.

l do not like to drive them. The ergonomics are bad and the car is big and has a smooth and quiet ride, so you do not feel the speed.

The BMW iDrive is also very annoying to use. Listening to the radio is complicated.

The BMW 5 Series did not do well in a crash test for SUVs and pickups. Which is certainly an issue they're going to have to address. But for crashes with other cars it did okay. That is really strange for BMW. The M5 is a completely different beast from the other 5 Series models.