It is difficult to evaluate the contribution of Mosby's raids to the overall Confederate war effort.
 In his memoirs, John Munson stated that if the objective was simply "to annoy the enemy," they succeeded.
In discussing as Mosby's "greatest piece of annoyance", the Greenback Raid in which Mosby's men derailed a train and captured a $170,000 payroll from the paymasters of Philip Sheridan's army (each of the 80 raiders received a $2100 share, though Mosby himself took nothing), Munson says that due to Mosby's comparatively tiny force 
... [i]t was necessary for the Federal troops to guard every wagon train, railroad bridge and camp with enough active and efficient men to prevent Mosby from using his three hundred raiders in one of his destructive rushes at any hour of the day or night. . .
General Grant at one point reported that seventeen thousand of his men were engaged in keeping Mosby from attacking his weak points, and thus away from active service on the firing line.
Finally it was not safe to send despatches by a courier unless a regiment was sent along to guard him.

On the other hand, Mosby's guerrilla operations were not highly regarded even within the Confederate Army.
Brigadier General Thomas Rosser (with the support of Generals Jubal Early and Fitz Lee) urged disbanding Mosby's command in a letter addressed to General Robert E. Lee.
Rosser agreed with the Union that Mosby's men were not soldiers but glorified thieves -- and bad for morale, because his regular troops were jealous:
[Mosby's men] are a nuisance and an evil to the service.
Without discipline, order or organization, they roam . . .
over the country, a band of thieves, stealing, pillaging, plundering and doing every manner of mischief and crime.
They are a terror to the citizens and an injury to the cause [because]
First.
It keeps a man out of the service whose bayonet or saber should be counted on the field of battle when the very life or death of our country is the issue.

Second.
They cause great dissatisfaction in the ranks from the fact that these irregular troops are allowed so much latitude, so many privileges.
 They sleep in houses and turn out in the cold only when it is announced by their chief that they are to go upon a plundering expedition.

Third.
It . . .
 encourages desertion . . .
they see these men living at their ease and enjoying the comforts of home, allowed to possess all that they capture . . . .
Patriotism fails in a long and tedious war like this to sustain the ponderous burdens which bear heavily and cruelly upon the heart and soul of man. . . .

General Lee sent the letter on to the Confederate War Department with an endorsement recommending "the law authorizing these partisan corps be abolished."
But the War Department simply reduced the authorized partisan commands to two, Mosby's and John H. McNeill's.
On later reflection, Lee concluded that whatever the military utility of the rangers in the larger scheme of things, Mosby was "zealous bold, and skillful, and with very small resources he has accomplished a great deal."
It is difficult to evaluate the contribution of Mosby's raids to the overall Confederate war effort.  In his memoirs, John Munson stated that if the objective was simply "to annoy the enemy," they succeeded. In discussing as Mosby's "greatest piece of annoyance", the Greenback Raid in which Mosby's men derailed a train and captured a $170,000 payroll from the paymasters of Philip Sheridan's army (each of the 80 raiders received a $2100 share, though Mosby himself took nothing), Munson says that due to Mosby's comparatively tiny force 
... [i]t was necessary for the Federal troops to guard every wagon train, railroad bridge and camp with enough active and efficient men to prevent Mosby from using his three hundred raiders in one of his destructive rushes at any hour of the day or night. . . General Grant at one point reported that seventeen thousand of his men were engaged in keeping Mosby from attacking his weak points, and thus away from active service on the firing line. Finally it was not safe to send despatches by a courier unless a regiment was sent along to guard him.
On the other hand, Mosby's guerrilla operations were not highly regarded even within the Confederate Army. Brigadier General Thomas Rosser (with the support of Generals Jubal Early and Fitz Lee) urged disbanding Mosby's command in a letter addressed to General Robert E. Lee. Rosser agreed with the Union that Mosby's men were not soldiers but glorified thieves -- and bad for morale, because his regular troops were jealous:
[Mosby's men] are a nuisance and an evil to the service. Without discipline, order or organization, they roam . . . over the country, a band of thieves, stealing, pillaging, plundering and doing every manner of mischief and crime. They are a terror to the citizens and an injury to the cause [because]
First. It keeps a man out of the service whose bayonet or saber should be counted on the field of battle when the very life or death of our country is the issue.
Second. They cause great dissatisfaction in the ranks from the fact that these irregular troops are allowed so much latitude, so many privileges.  They sleep in houses and turn out in the cold only when it is announced by their chief that they are to go upon a plundering expedition.
Third. It . . .  encourages desertion . . . they see these men living at their ease and enjoying the comforts of home, allowed to possess all that they capture . . . . Patriotism fails in a long and tedious war like this to sustain the ponderous burdens which bear heavily and cruelly upon the heart and soul of man. . . .
General Lee sent the letter on to the Confederate War Department with an endorsement recommending "the law authorizing these partisan corps be abolished." But the War Department simply reduced the authorized partisan commands to two, Mosby's and John H. McNeill's. On later reflection, Lee concluded that whatever the military utility of the rangers in the larger scheme of things, Mosby was "zealous bold, and skillful, and with very small resources he has accomplished a great deal."
43rd Battalion, Virginia Cavalry