I take photos with my DSLR but as Genius1 has already stated most of my points, I always let the crew know that I will be taking pictures of some aspects of the flight, cabin and service but I always make it a point that no stranger is identifiable in my pictures (especially on trip reports).

I try and be discreet too and have never used flash at any point (this is definitely a challenge on a night flight!).

/AE