I only started experimenting with Customs Races at the weekend, and it was pretty good. Several things that I dislike about Campaign Mode are rectified - getting rid of the boost is a major, erm, boost, while standing starts and damage add to the realism of the experience. The amount of Cr. and EXP available seems quite light, but on the other hand, it is not too hard to find a set-up that gives a decent challenge - it seems to come down to picking your spot on the grid sensibly. On Pro difficulty, I cannot start last and win a 12 lap race, but starting nearer the front (e.g. 5th) made it too easy. Ironically, the predictability of the AI makes it a lot easier to figure out how to give yourself a decent race, but it does mean a few goes to figure out what kind of average lap time you need to be hitting in order to have a chance of winning.
I haven't ventured into Sport mode, mainly because I don't like it (waiting on races to start, lag, idiot opponents etc.), and now I don't have PS Plus anyway, so it's not an option for me now anyway. I want to concentrate on making Cr. and buying cars, but Campaign Mode is fairly limited - Custom Races offer a way to do what I prefer doing, i.e. playing offline, while also offering a way to make Cr. without grinding the same races over and over again.