If you ate only cheese for a week, you would definitely not be feeling well, probably by Day 3. All the dairy and sodium would make you feel sluggish and constipated. You would probably gain weight from your body having a lack of nutrients and being stopped up.
As for fruit, you would be going to the bathroom a lot more but it would be difficult to feel full, plus they have a lot of (natural) sugars in them so it's not necessarily your best option for losing weight. You might lose weight but as soon as you start to eat normal foods you will gain it all back.
There is no miracle food but if you are going for a healthier diet and want to gain lean muscle and burn fat, your best bet is to eat a balanced diet of foods in their natural state (aka not processed, unrefined foods). Eat a portion of protein and a carb at every meal.
The key to losing weight is keeping track of your calories and making sure you are staying about 500 calories short of the amount you need to maintain the weight you are at. Gaining muscle means you have to up your protein levels. I use caloriecount.about.com to keep track of how many calories I eat, burn, and the percentages of my food that are protein/carbs/fat.
Hope this helps!
http://caloriecount.about.com/ - for a calorie counter