Yes, I would use it. I'd use it to go back to Saturday, the 24th of July, 2010.
That was the date of The Prodigy's Warrior's Dance Festival, aka the most epic night of my life. I'd re-live from the morning, not have those fucking awful chips I had so I wouldn't vomit halfway through the night, but apart from that I'd just re-live it.
Then, life would continue as normal. I'd do everything I usualyl do, except I'd be keeping the motivation and knowledge I have now to lose weight and get muscle, so I'd be starting early. Back then I had no motivation to, but now that I've met a special someone, I do.
By the time I re-meet her, I would be a better person than I was, and I would know what chances to talk to her come up, and what I should say when they do.
Therefore, everything will go a lot smoother. Being less fat than I was, and therefore more attractive, and also being more confident due to already knowing what to say and what not to say, the relationship should develop far more than it is currently. And definately not accidently let slip to a certain dickhead my feelings for her, because I'll know he'll pester me about it and drop hints about it when other people are around, when I wanted it kept secret.
As a bonus, I'd know what to revise and what not to revise for my final GCSE exams. Would save a lot of time.
Also, it would be really cool to have the surreal experience of knowing what's going to happen before it does...