For the most part, re-using plastic water bottles is fine. The only concern is bacteria grows in water very rapidly. So ALWAYS wash them after use and keep them refridgerated. Never leave them in your car on warm days and if its necesary to do so, keep them in a cooler. You must remeber everytime you sip on your water bottle, bacteria from your mouth is getting washed into the bottle. Moist, warm environments are bacteria's best friends.
Cheers