Hi,
Semi-open is what I prefer to open and closed. I only
owned the
semi-opened AKG and several cheap opened headphones myself.
I personally own a Bayerdynamic DT-770 Pro and I'm very happy with it.
Of course you need to know the frequency-curve and mixing (studio)
should be done with more than one headphone (and speakers!) but for
live-mixing this thing is superb! And I never had a reliabilty-problem
with it.
If you're looking for a semi-closed one you can take a look at the
Beyerdynamic DT-880 Pro which is the semi-closed variant of the
DT-770. It's the most expensive in the series. The DT-990 Pro is the
"open back" variant and costs about the same as the 770.
Best regards,
Jannis