I use a 'temporary' cover (over exhaust fan) to prevent falling leaves to enter.

Using a full cover can create a nice cozy spot for critters to winter-over.

Outdoor units need to breathe.`-+

Milehighdiva

On the surface those covers seem like a no brainer.

However, I agree with V.

A compromise that I like is to get a sized piece of plywood to cover the AC condenser top and place a cinder block on the plywood to stabilize.

This stops the leaves, snow, and ice yet let's the condenser breath without collecting condensation.

Just an idea.

IMO

In most climates, covers on anything outdoors, cars, AC units, patio furniture, need to be able to release moisture at the top of the cover. Water vapor is less dense than air so it rises into an enclosure and concentrates. If the covers can't breathe, you get a rain forest climate going in there. Excessive moisture accumulates and can cause corrosion.

Thank you for the replies! I might try this: http://www.amazon.com/CONDITIONER-COVER-Season-Black-Premieraccovers/dp/B003CUPH8K/ref=sr_1_11?s=home-garden&ie=UTF8&qid=1319757746&sr=1-11

It only covers the top and it's mesh. But, before I order I'll measure the unit and see how much it will cost for the local hardware store to create a plywood top as suggested by tigerdune.

You might check your local home center. I have seen odd- sized sheets of plywood or strand board (much cheaper).

V