If you usually main in melee classes then this might be a bit costly for you as a lot of core skills for melees have either low drop rates or can only be bought in the store. If you're okay with range classes (engineer/rogue) then you can reach the full potential of your character without cashing in. The only draw back is that it would take some patience to grind for those skills book pages but yeah it's very doable.

I've seen F2P rogues outplaying barbarians and swordsmen with a 20-30k CP difference in PVP. So, it's not totally play to win, there's also a factor of your reflexes. I've spent around $25 in the game because I want to gamble in the gatcha (to get high level equip and familiars) and to open up some storage spaces. Later did I realize that you can get alot of them for free as you progress through the game.