Maintenance

Part of any evaluation of an airplane includes what maintenance issues the type has. No plane is perfect and the CJ-6 has a few minor gripes.

Exhaust Pipes

The original pipes don't last very long. Carbon Monoxide could leak into the cockpit. There are stainless steel exhaust pipes available.
Doug Sapp provided some more information:
It has been our experience that the main problem with the short amount of time we can expect to get out of the system is caused by our lack of usage. In China where these aircraft are flown continually day after day these sustems last for a long, long time. Because we fly so little the systems rust away on the insides and soon fail. the failure mode is normally first the lower left, then the lower right then the upper left. The balance of the system lasts for many, many hours. To solve this problem I had the factory in China make me entire systems out of stainless steel. These systems are made on the very same forms as the old steel systems so every part is interchangeable. What this means to you is that you can change the "wear" parts to stainless steel as they fail because the stainless parts are 100% interchangeable with the stock steel parts. This saves you a bunch of $ and prevents you from having to throw away the remaining perfectly good exhaust parts.
About exhaust gas from the leaking exhaust entering the cockpit:
This is largely a "wives tail", the exhaust gasses which enter the cockpit are coming in via the rear canopy skirt and the opening in the floor boards via the wheel wells. In flight the interior of the cockpit is a low pressure area when compared to the pressure of the gasses and air flowing across the aircraft canopy's an aircraft skin. Because of this any exhaust gasses in the area are quickly sucked into the cockpit. A ram air (for summer use) will help to make the inside and outside pressure close to equal will stop the gasses from entering the cockpit. A exhaust system with a cabin heat muff will help do the same thing during the winter months if your flying in the colder weather. Sealing the cabin floor and the wheel wells helps also, but the problem will never be totally solved until you cause the cabin pressure to equal or exceed the outside pressure.

Asymetric Fuel Use

The fuel from the wings flows to a header tank in the fuselage prior to being used by the engine. There is no fuel selector valve, just on or off. The pilot can use cross controls to raise the heavier wing to encourage the fuel to feed. A modification is available to close the fuel vent in the wing tanks to allow the fuller tank to feed.

Hydraulic Lock

More of a issue with round engines in general. Oil collects in the bottom cylinders and can cause damage on starting. There are several mods that address this issue. It's also possible to remove the plugs and drain the cylinders of oil. http://staff.jccc.net/droberts/yak52/m14p_fa.html has a good discussion on radial engines.

Pilot Height

Pilots with excess body length could have trouble fitting into the cockpit. While pilots 6' 2" or so report no significant problems, there is a replacement "bubble" canopy available.

Pneumatics

Not a problem so much as it's just a little unusual here in the States. All reports is that it's a very reliable system. The system can be charged from an external source.

Spark Plugs

Original spark plugs have become quite expensive. Fortunately, an automobile conversion has been developed. The price of the conversion kit is about the same as a set of original plugs. An added benefit to the conversion is your engine runs better!

Nose Wheel Shimmy

Some CJ's experience a nose wheel shimmy. There is a Cleveland Nose Wheel modification available.

Parts/Maintenance

http://www.cj6.com is the primary source of parts on the west coast. I understand there's a company in AZ that also sells parts. http://www.nanchangaircraft.com is the premier service facility on the west coast.