At least on my machine, OpenGL seems to run significantly faster and the shinemaps also look nicer. There are no problems with the interface on the standard 1024x768x32 mode and I haven't encountered any stability issues, which is a refreshing change considering that my computer is currently quite messed up in other ways. D3D used to be faster in 3.55 but is quite sluggish in more recent builds, especially when there are lots of ships in view.
Any ideas on when the next release version of fs2_open is coming out? I am holding off testing my PI missions at the moment since there are several different versions floating around at the moment that each have their own features.
By the way, a new problem has come up in the launcher recently; it seems that it does not remember any changes made in the video tab and always reverts the settings to the default D3D5 whenever it is started, so I have to set the stuff there every time I start the launcher. Not a major issue, but it can get a bit annoying.