Depends which 109. In the E-4, it's automatic by default. It helps keep your attention off the engine and on your situational awareness but ultimately is not as good as a true CSP.

At the very least, in a sustained climb situation you're better off using manual. In combat I would suggest auto, but there are lots of 109 guys who use manual all the time.

Thanks - your help is much appreciated.

BF109 E-4/B models has automatic prop-pitch. If you look at the panel you can see the prop-pitch handle has been removed for these two models. The earlier models has manual pitch.
I like like the auto-pitch as it will maintain speed at all times letting me focus on other tasks in combat. If you really know how to handle the pitch in all situations you might be better off with manual though i have not really tested this or seen any tests comparing auto vs manual.

Raggz, the manual proppitch switch isn't gone in the E4/E4B, it is just moved to the throttle and a bit smaller, to be used with the thumb.