Zhuge Liang was certainly a capable commander. In his northern campaigns, the Shu army usually took the initiatives and tried to force field battles against the more numerous Wei army. The Wei commander, Sima Yi, was forced to hold out in his defensive positions until Zhuge Liang died in his last expedition. However, his campaigns all ended in failure, mainly because of supply line could not be maintained. Though he did find more success in putting down rebellions in the south.

I would also note that Zhuge Liang was not a prominent commander under Liu Bei, who assigned mainly administrative and diplomatic work to him. Zhuge Liang was part of the reinforcements to invade the Yi Province, but not much was recorded about his battles. It was Fa Zheng who urged Liu Bei to take over Hanzhong and served as advisor in the campaign, while Zhuge Liang stayed in the capital, Chengdu. He again stayed in Chengdu when Liu Bei suffered disastrous defeat at Xiaoting against Wu. Liu Bei clearly valued Zhuge Liang as an administrator and tasked him to maintain supplies and reinforcements while at war, but rarely seeked Zhuge Liang for advises on military strategies and tactics.

The Romance of the Three Kingdoms is a novel which is both historically accurate in some areas and fictionally fantastical in others. It takes information from reliable accounts such as 三国志 (literally translated into 'The records/annals of the Three Kingdoms') and 世说新语 (literally translated into 'The new recounts of the World'), so we can conclude fairly safely that it isn't all just made up.

On the subject of Zhuge Liang - we have sources other than the Romance of the Three Kingdoms that attribute certain inventions to his name, such as the 馒头 (steamed bun), the wheelbarrow, and improved the repeating crossbow.

We also have access to texts composed by himself, such as 三十六计 (36 stratagems - a book that outlines certain stratagems that could be used in dealing with others, in war, or in politics) and Mastering the Art of War (which further develops the ideas around the famous 孙子兵法, or Sun Tzu's Art of War).

This then proves to some extent that Zhuge Liang was, to some extent, a brilliant strategist - however, as a commander, not so much. While there are some historical inaccuracies or large exaggerations of his prowess (borrowing arrows from the boats comes to mind), other strategies have been recorded (e.g. the Empty City). Read below.