fruit are a dieters friend imo (unless you are lo-carbing i think )
Yes you can overdo anything as long as you eat reasonable amounts, they are very healthy.
the only fruit i would not eat freely is avocado which has a lot of fat - one florida avocado has almost 500 calories!
Also fruit juices and dried fruits have alot of calories also.
Stick with fresh citrus, melons, berries and other fruits and you will take in less calories and get more vitamins and fiber too! A good easy idea is too switch from high calorie snacks and or desserts to fruit, you can eat more and still save on calories
Hope this helps