It is my opinion that Lend-lease was vital to the existence of the USSR in WW2. Not from the perspective of AFVs and aircraft - the Soviets provided the vast majority of those items for themselves and in any case, the Soviet built items were generally more effective than the US/UK built ones.
The real area where Lend-Lease made all the difference was in logistics. For example, the US supplied 429,612 soft-skinned vehicles to the Red Army. This aid was vital in allowing the Red Army to transport logistical supplies and troops over large distances. It is said that the existence of Lend-Lease allowed the Soviet soldier to have at least one meal a day.
Moreover, there is evidence to suggest that Lend-Lease also kept the Soviet railroad system operational. Without this, it would have been very difficult for the Red Army to move large numbers of men and equipment around, thus making it very difficult to organize large-scale operations. For example, Lend-Lease supplied 92.7% of railroad tracks and 81.6% of all locomotives.
Without Lend-Lease it is my belief that the Soviet Union would not have the necessary logistical infrastructure in place to repel the Germans. In short, it proved instrumental in their eventual victory. Large scale operations such as the counter-attack at Moscow, Stalingrad, Kursk and Operation Bagration to name some would have been impossible to mount otherwise.
None other than Gheorgi Zhukov himself is on record stating that Lend-Lease was 'indispensable' to the Soviet war effort.
http://lend-lease.airforce.ru/englis...ents/index.htm http://www.o5m6.de/Numbers.html http://orbat.com/site/sturmvogel/SovLendLease.html