Certainly so - he had many opportunities to crush the enemy as Lincoln requested, but his timidness let him down time and time again. I recommend James M. McPherson's Battle Cry of Freedom to display his numerous faults as a commander.

For example he had an opportunity to make an assault upon Richmond during the Peninsula Campaign, there would have been a strong fight to be sure, but he had the numbers to take on Confederate Defenses, he simply failed to do so due to characteristic knack for overstating the size of the Confederate forces.

The same applies for the Battle of Antietam, which could have been transformed into a decisive Union Victory, or at least a greater victory than it was, had McClellan been more decisive in assaulting the Confederate positions, the destruction of the Army of Northern Virginia could had been achieved. McClellan did not attack at all on 18th September, allowing Lee to withdraw across the Potomac - out of McClellan's grasp, even though Lee had lost around 30% of his Army, (McClellan had lost around 25%, but had far more men at the time anyway).

So I certainly feel that McClellan prolonged the US Civil War, however, had it not been prolonged Lincoln might not have been pushed towards the Emancipation Proclamation, and the resolution of the war at an early stage would historically have been a negative consequence in the development of the US, if there was no settlement of slavery the issue would continue to divide the US for years after, and the South would likely have remained backwards - so I suppose every cloud has a silver lining, at least in this case.

Source: Battle Cry for Freedom

McClellan's Civil War career went like this:

• 4/61-8/61: Semi-effective command in what would become West Virginia, winning a couple of small battles
• 11/61: Elevated to general-in-chief
• 11/61-3/62 Trains the Army of the Potomac and holds in place around DC while Grant wins several major victories in the west; McClellan frustrates Lincoln during this time. They argue over strategy. Lincoln wants an immediate move towards the Rebels at Manassas. McClellan plans an elaborate flanking move aimed at Manassas. Lincoln overrules the so-called "Urbanna Plan." Then the Rebels evacuate Manassas and it is revealed that their fortifications were bolstered by "Quaker Guns"--logs painted black. Derision ensues.

If McClellan is not using the army, I should like to borrow it for a while - A. Lincoln

By some strange operation of magic I seem to have become the great power in the land…were I to win some small success now I may become Dictator or anything else that might please me—but nothing of that kind would please me—therefore I won’t be dictator. Admirable self-denial! - G. McClellan

• 4/62-6/62: The Peninsula Campaign. McClellan sails his army from Alexandria to the Virginia Peninsula and marches overland towards Richmond. He is slowed down considerably for a variety of reasons--his surveyors misjudged the firmness of the roads and his intelligence officers vastly overestimate the forces opposing him. He fights and wins several lopsided battles and eventually ends up within sight of Richmond, but taking ~90 days to travel 45 miles is not ideal in the Civil War era.

I am tired of the sickening sight of the battlefield with its mangled corpses & poor suffering wounded… Victory has no charms for me when purchsed at such cost - G. McClellan

• 7/62 - the Seven Days. Lee takes control of the forces around Richmond and attacks McClellan for a week straight, driving him away from the capital. McClellan "changes his base" of supplies to the James River, which in the context of the campaign is essentially a retreat. It becomes a Southern joke that when a gun fires and a scared dog runs away, that dog is only "changing his base."

• 8/62-9/62: With McClellan cowed and evacuating the Peninsula, Lee moves north and defeats another Union army at Manassas before invading Maryland. Although he might have been dimly viewed based on his conduct on the Peninsula, McClellan is still the obvious choice to take command of the pursuing forces. McClellan's pursuit is quick and effective, and it is made all the more quick and effective when some Union soldiers find Lee's campaign plans discarded near a Confederate headquarters. McClellan breaks through Lee's blocking force at South Mountain and fights him to a tactical stalemate at Antietam, although the battle represents an operational and strategic victory for the Union. McClellan does doggedly pursue Lee (many people forget about this) but several stubborn Confederate rear-guard actions allow the remnants of Lee's army to escape into Virginia intact.

• 10/62-4/64: After Antietam, Lincoln issues the Preliminary Emancipation Proclamation, which raises some hackles in the Union Army. Being a Democrat, McClellan is also discomfited by the PEP, and tells the soldiers under his command to "make their feelings known at the ballot box" or something like that. Soon after, Lincoln removes him from command for the rest of the war.

• 11/64: McClellan is Lincoln's primary opponent in the 1864 election. Although McClellan says that he will not seek peace with the Rebels, it is part of his party's platform for almost the entire election. That seems to be a winning electoral strategy until major Union victories in August, September, and October buoy Lincoln to a landslide re-election.

My conclusion is that McClellan was not as good a general as Lee or Grant, but he was no worse than any other army commander (McDowell, Pope, Burnside, Hooker (maybe Hooker was better), Meade, Rosecrans, Sherman, Butler, Bragg, Pemberton, either Johnston) that served during the war.

Did he prolong the war? He had a good shot at capturing Richmond in the Peninsula Campaign, but he moved too slowly. That probably would have shortened the war. It would have been very difficult to have destroyed Lee's army after Antietam, and he did mount a solid pursuit, so I don't fault him for that.

This is, of course, a highly abbreviated history of McClellan. I'm happy to go into more detail on any point.

You are fortunate in that one of the best historical authors of recent decades, Stephen Sears, is a McClellan-o-phile. I would check out his "To the Gates of Richmond" (Peninsula), "Landscape Turned Red" (Antietam), or "The Young Napoleon" (McClellan biography) for a lot of great material.

Others have answered the question well, but I thought I'd mention that even among his contemporaries at the time there were divergent opinions on McClellan. Grant somewhat famously declared that "McClellan is to me one of the mysteries of the war", indicating that he didn't know how to feel about the man either.

Source: Battle Cry of Freedom, McPherson