Let's assume that when you fast forward or rewind time, you retain your mental state and your ability, but your age and location are changed to whatever they were at that point in time.

With this ability, you could know everything that would happen. You could go back and fix any mistake you'd made. You could keep rewinding the same day over and over and read the entirety of a city public library. You could relive your favorite moments as many times as you pleased. You could ace every answer on any game show. You could jump off a skyskraper just to see what it's like (without ever hitting the ground, of course - your ability isn't of much use if you're dead). You could know just the pickup line to use on any given girl.

Now, if you were able to start and stop time, you'd be the ultimate pickpocket, the most mysterious and capable fighter in the world (your opponents would crumble, beaten half to death before it appears you've even touched them), and the fastest man on earth. Not much else, I think. Ogle girls in the shower? Psh.

Well, rewinding equals immortality...

the first is a good thing
the second, other people would lose all their stuff and hackers would rule the world

don't want that

When I'm reincarnated, will I have my memories?
If so, I choose that, if not, time control