I'm also doing a GY at the moment. I haven't read what everyone else has said, so I won't bother going through question by question to avoid repeating everyone else said.
If you are considering a GY, make sure you have planned it well in advance. There are loads of positives to taking one, namely getting a decent amount of money, travelling (and spending that money!), getting some work experience, etc, but if you don't have a plan from the start it will become pretty dull.
I had a ten-month internship lined up at a Bank, which fell through due to lack in funding on their part. By this point, I hadn't applied to University, and so had little choice but to take a GY anyway. This would be fine, I thought.., as I could get some good work experience and there were about 6 of us planning to go travelling later in the year, and they lived close so I could regularly see them. On results day, however, these guys made the decision to go through Clearing and go to University this year. I hadn't really got a back-up plan. All my friends have gone to Uni now, and I'm not going to lie to you it's pretty lonely. I live quite close to Manchester, and can visit friends there to go out but I miss those day-time interactions at College which I didn't appreciate at the time, and it can be quite boring. I've had some really good work experience, and am starting a job soon, which takes your mind off it as there are obviously people to speak to there. After May, I plan on still going travelling, with the same people, but it wasn't the original plan.
In short, make sure you have two plans in case the first falls through. As long as you keep yourself busy it will fly past, but if you're just sat at home alone, it will be quite a lonely year..
Perhaps the biggest positive though is the fact that you will have friends at all the Universities you want to go to, and so can ask first hand questions! The worst part is freshers week and seeing all the photos on Twitter/Facebook/SnapChat..