[wikipedia]Nissan has issued several recalls on early QR25DE engines relating to excessive oil consumption. The most notorious example of engine oil consumption recalls relates to 2002 Nissan Sentra SE-R Spec-V vehicles. Recalls noted failure of the engine exhaust manifold catalyst due to excessive oil consumption leading to contamination of the catalyst and failure. Nissan replaced QR25DE engines under recall often more than once on the same vehicle. The cause of engine oil consumption is due to a combination of closer piston ring placement towards the crown of the piston, overheating of the piston rings, failure of piston rings to seal correctly, and long stroke distance of the piston within the cylinder. Contamination of catalyst substrate with oil eventually causes the catalyst to ignite and restrict exhaust flow, the result is exhaust backpressure increase along with overheating of the exhaust system and exhaust connected side of the engine.[/wikipedia]
I know about it first hand, replaced one engine at 40000km, and another one 20000km later.
The Nissan VQ40DE (4.0 V6 in the Frontier/Navarra, XTerra, previous-gen Pathfinder) is a pretty good engine as well. Traded in my '06 Xterra with 262,500km on the clock and it had never had anything done to the engine aside from routine maintenance. AFAIK, the engine's only real weak spot are timing chain guides that become brittle and eventually fail and allow the chain to jump with rather unpleasant results. I've heard that the rest of the VQ line of engines (powering the 350Z, 370Z, various Skylines, and many other Nissan and Infiniti models and even a few Renaults and Suzukis) are also very well thought of.