You shouldn't have bought Rubik's brand ones. They're all horrible to turn and the stickers suck and peel in no time. I know as they were the first ones I bought.
Now I have about 60+ twisty puzzles, including a Teraminx (google it) which I've only solved twice, as it takes about 4 hours.
Shengshou make lovely 4x4 and 5x5 (and all the way up to 9x9) cubes if you ever fancy some. Once you can do a 5x5x5 you can solve any size cube.