I know there have been threads stating that spoilers now affect traction slightly since the Hipster update (I can't find them with search) but it really does have a big impact on the game. If you didn't already know, adding any spoiler to a car now adds traction - all to the same level, bigger spoilers don't = more traction.
I've never been a fan of spoilers on most of my cars but now it's almost a necessity, definitely if you plan on racing. It really "spoils" the look of my Massacro and Vacca!
It's more than just a slight improvement in grip, it affects the overall character and drive-ability of many cars that may alter your views on them. I never got on with the Elegy for example, I could definitely get my fastest lap times on most tracks in it but I didn't like its grip-grip-grip-spin handling. I didn't find I could drive it consistently and much preferred the Jester or Banshee. Like most modern cars handling is sacrificed for sheer grip, which is great but when it eventually breaks away it's hard to counter, it's not progressive. Now I find the Elegy far more consistent and I got my fastest lap on Cutting Coroners in it this morning (55.2), that was until I put a spoiler on my Zentorno (now 54.8!). The Elegy is now easily my favourite sports car. My Vacca now has immense grip in the corners and with its bullet proof rear will be my go-to city driving car. It might be worth revisiting some cars you may have written off like the Comet which was always just a little too tail happy but now may be more easily balanced too.
The one drawback is that now everyone will have to put spoilers on all cars if they want to be competitive, I tend to think it would be better if they worked more like real-life and lowered top speed a little as a side-effect, making them more of a choice depending on car or track.
Overall it's great that Rockstar are adding more "average" and interesting cars and updating things like the spoiler effect (though if I were cynical I'd say it's so we have more to spend our cash on!) so it's certainly a step in the right direction and adds more interest to the game. Maybe tyre/wheel types are next, who knows!