I presently own two pairs of headphones that could be considered "semi-open" (and have owned a third in the past); I wouldn't wear any of them outdoors. I had a pair of SR-60s that I used outdoors a time or two; I liked being able to "hear-through" to the world around me, but I disliked being able to "hear-through" to the world around me...if that explains my position.
It's nice to be able to hear what's around you in case you need to react to something quickly, but if you're going to use it on public transit, you might want to consider whether other people want to be able to hear your music clearly.
That's mainly why I only use earbuds or closed headphones in public.
Ive worn the superlux 668b in public, on the one hand its good if you are walking around and need awareness, however if you are actually trying to listen/analyze the music the outside noise can be distracting. It isnt that bad, and I sat inside the the center of the Trinoma mall (which is a huge wide open area with maybe 1k+ pedestrians, musak and random loudspeaker annoucements) and I enjoyed them, the only other HP i had while travelling was the HD 201 and I preferred using that if outdoors + stationary.
I remember one time trying to use a px100-ii and HD 515 inside a fast food restaurant and I couldnt hear my HPs it over the overhead musak. Trying to listen to fully-open phones in public really showed me quick why people value isolation.