I don't care how good looking or well made condensing unit covers are, they are a bad idea. As has been mentioned; putting a cover on allows moisture to build and attack the components under the cover and gives a safe haven for rodents and bugs.
Equipment manufacturers go to great lengths to make condensing units and heat pumps that hold up under the elements. That is an automotive paint finish on the unit, treat it as such. Wash and wax your condenser/heat pump just as you would your car.
If you have problems with debris or ice from overhangs possibly damaging your unit you may want to think about doing something to divert the damaging materials from your unit. Outdoor units are not supposed to be installed under overhangs unless the overhang completely protects the unit and is high enough to allow for proper air flow.
If you absolutely must protect the unit with a cover, only cover the top as has been shown and described by those who have stated how to do this. A crappy looking piece of board that protects your unit is much better then a nice looking cover that may allow moisture, rodents and bugs to damage your unit.
Ever see what rodents do to wiring?