The "flat-back" design isn't really effective. When the barrel fills up with water, the back swells out.  This causes the barrel to push (about 2 inches)  away from the wall.  It's best to set the barrel 3 inches from the wall.  This swelling also causes the rectangular filter opening (on top) to warp.   

During a heavy rainfall,  the overflow spout cant handle all the water.  Thus,  water just pools on the top of the barrel and pours over the side. 

Be careful installing the spigot.  Instructions say to lay the barrel on it's back (to make it easier).  I did that but the spigot still didn't go in flush.  Once the barrel filled with water,  the spigot leaked.  I had to apply a waterproof sealant to the threading to stop the leak.