I've received mine last week.
I've tested RiscOS. Seems to be a nice OS on Rasp when completed !
I use Raspbian on a daily basis.
Good points :
Quake 3 compiles and runs perfectly
All demo sources codes (in /opt/vc) compile and run great .
We don't need X to display gfx ! That's a good point for programming this fruit.
broadcom's gl libraries seem to be of a good quality.
Weak points :
memory footprint with raspbian is quite huge for such a tiny device (between 40 and 50 Mb)
We need of a good ARM assembler. AS could do the job, but it's not ARM syntax compliant.
If you want to use 3D functions, a memory split of 192/64 or 128/128 is needed.
I wish someone releases an asm-one like environnement. In the meantime, emacs do the job.