1.  Don't buy the cheapest one you can buy.  Look for a rotating construction that is easy to use to use, accurate, can handle getting wet and can be serviced if needed.

2.  Always purchase an automatic leveling laser. For example a Leica Rugby 610 or a Rugby 810 laser. The manual lasers are a real hassle and will just end up getting you in trouble on your jobsite.  Auto leveling lasers are the only ones you should buy.

3.  When you purchase a construction rotating laser, purchase a good tripod for the laser.  Just setting the laser on a bucket, is an accident waiting to happen.

4  Look at the type of batteries that run the laser.  Are they only alkaline, nicad or li-ion batteries.  Can you buy a auto charger to charge up the batteries?  A car/truck charger is a great accessory that many contractors really like.

5.  Service and follow up down the road are very important when you purchase a construction rotating laser.  A good laser dealer can check calibration for you if your laser is mishandled or dropped.  That can save you both time and money down the road.