I had a 109E up against a Spit V the other day in the TA and couldn't believe how well it could hold it's own in the turn-department. After flying lots of overweight 109's, the E felt positively light!
Not saying it has any great attributes, but the fact that it doesn't get used much is more is just as much a matter that it has 4 older siblings to choose from. It's not a total lack of capability. Sure, it's kinda slow, but not more so than a Spit V, F4F, FM2, etc... the cannons are pretty tricky to fire, but when they hit they get the job done. The cowl guns, well, you can tickle people with them.
-Soda
The Assassins.