I tried the 1940 Barbarossa against the AI once. I didn' honor the M-R pact, and then moved into the Baltic States before the Soviets annexed them so as to get an even more forward position. It was a fun game but there were several problems:
- Germany doesn't have time to tech up by summer of 1940 so you're going in with little or no tech advantage over the Soviets. That means a slow sluggish advance.
- You don't get the huge MPP boost from the fall of France.
- You don't really save any occupation troops in the West because you have to man the Siefried line.
- Without the French ports at your disposal, using your subs is even trickier than it usually is.
- You'll be going in without the help of Hungary, Romania and Bulgaria. Sure they'll join later on, but those extra troops could sure help at first.
One good thing is that there are no severe winter events scripted for 1940, meaning that you don't have to worry about supply and strength suddenly collapsing.
EDIT: Oh and the french navy is also an extra problem you have to deal with in this case.