Being at a healthy weight and having a good level of fitness feels great. If you're overweight it'll certainly take some work to get there first. But after that, with the changes in lifestyle you'll have made, it's not too hard to stay where you are. You can then eat sensibly (i.e. not junk food every day, but a mixture of healthy things and treats) and exercise maybe 1 hour 4-5 times a week, and easily stay at a good level of fitness.
Really there's no down side to it for me! I like exercising anyway, and your body looks better just as a by product of what you're doing. Plus you feel more energetic and so that and, to an extent, the improvement in the way you look, will have you feeling better mentally too. Even if you don't like exercise, it's well worth it to put in a few hours a week to get years of higher quality living.
The cost isn't too high either. I do have a gym membership, which is around £25 a month. But I could easily get by without that if I was poor. A few weights and somewhere to train outside would work. If you're incredibly busy then time might be a factor, but most people have at least a few spare hours in their week they could use for exercise if they wanted to. When they'd otherwise be watching tv or talking about things on the internet...