If you actually play the game (i.e. not run around only in Freemode or do the same race/mission over and over and over again), it really isn't that bad. Those who complain about the grind to 100 are those who aren't exactly doing anything else. Of course it's going to feel like a grind when you only do the same two or three missions (less than 1% of all that can be done)!
It takes about 10 rank-appropriate missions (the rank required to get it being about the rank you are) to rank up one level. But if you find people to play with that are higher up and play their missions, early on you can rank up every 2-4 missions (or even every mission really early on).