I wouldn't say he is overrated at all, hes by no means the god of war, but still one of the best German commanders of the war if not one of the best period. Some of his accomplishment were:

"Sickle Stroke"- arguably his most famous work, and possibly his greatest contribution to the German war effort. His revised plan for the invasion of France called for the armored thrust into the Ardennes that was responsible for defeating the French so quickly.

Next Manstein was transferred to a panzer crops at the start of Barbarossa, Manstein's corps fought pretty well and Manstein earned distinction, but he had a habit of outrunning his supply lines and almost got his panzer corps destroyed when he got encircled by the Soviet 11th army.

After this Manstein was transferred to the German 11th army under army group south in the Ukraine. His goal was to capture the Crimea and take Sevastopol. Manstein slowly worked his way into the peninsula but due to lack of manpower his assaults on the fortified city failed. He was able to stop a Soviet counterattack from taking the eastern part of the peninsula though. Manstein eventually got the air superiority and support he needed. He took both Sevastopol and another city on the peninsula called Kerch. Hitler promoted Manstein to Generalfeldmarschall because of his victory.

He was than transferred to the city of Leningrad, where he made the decision to starve the city by cutting of the supply route across lake Ladoga. However, the Soviets launched the Sinyavin Offensive, and while Manstein was able to repel the assault, the Germans were to weakened to launch a final assault on the city and they had to commit to a siege.

Manstein was now given the task of reliving German troops in Stalingrad (operation Winter Storm). Manstein intially thought the 6th army in Stalingrad could hold out and he did score some victories, but eventually he realized the need for the 6th army to break out of Stalingrad but Hitler refused.

The defeat at Stalingrad opened the floodgates and the Soviets pushed back hard. Manstein launched a counter offensive that was extremely successful destroying three Soviet armies and preventing the collapse of the entire front. He even recaptured the city of Kharkov.

Manstein's first real defeat would come at Kursk. Manstein did cause heavy casualties but, the allies had landed in Sicily and Hitler began to pull some of Manstein's troops away. This angered him as he felt that another push would destroy the Soviets. In truth there was no way that the German's could have continued fighting.

Now the front began to collapse and the Germans began to lose territory rapidly. The Germans just didn't have enough men to cover the front and Manstein's counter attacks did very little. This continued until Manstein was dismissed in March of 1944.

Overall, he was a very competent general, and while he made a few mistakes, he was more a victim of circumstance as the Soviets just held too much of an advantage for him to do anything.

Sources:

The Second World War : John Keegan

The Third Reich at War : Richard Evans.

Lost Victories: The War Memoirs of Hitler's Most Brilliant General (his memoirs.)