IMHO the Gr 3 Road Cars are worth it... the Special Safety Cars not so much...
Still, remember they are available on the Mileage Exchange on cycle, not random, so any car will be available to buy again in 16 weeks time (8 cars * 2 weeks cycle)
You must prioritize how you should spend your Mileage Points:
- Car upgrades (basically useless for Sport Mode, because of BoP, not really essential in Campaign Mode also...)
- Paint chips and wheels (if you intend to have fun in Livery Editor right from kick off, go for it...)
- Helmets, suits and poses (like paint chips and wheels, if you are in to that...)
- Special cars (the Gr 3 Road Cars are awesome in many Nxxx categories, as they have a lot of downforce compared to "normal road cars", refined suspension and more rigid chassis, the Safety Cars, some people love to put a police livery and accelerate through Tokyo Expressway just for fun and/or Photo Mode, depends again to what you would like to do). Just remember that, contrary to Gr 3 Road Cars, that are N500 cars, the Safety Cars are Gr. X cars, so may be ineligible to some road car races or its power output deficit would not matter to get a Handicap Bonus.
If you primarily play Sport Mode, no not really. I have all the available mileage exchange cars because you get to a point where there's so little so spend mileage points on you may as well. I only have each car in one colour as I'm not bothered about owning the same car in different colours.
And that too