You can cut branches that grow over your property line. But in the process you should not harm the tree. This basically means you should prune the tree at right season like late winter or very early spring. If it is a fruit tree, the English common law says fruits belong to the owner of the tree even the branches is over property line.
For the leaf since your neigbour can not enter your property to collect them, it's your responsiblity.
Does it look good? My rear neighbor plant shrub to seperate the property. They looks nice. I like them, they looks like part of my yard too. I don't lose one inch of property and pay nothing. I do have to collect leaf at fall and trim a little if they shade too much of my grass.
