well from my experience of it, obviously the ball is alot smaller and because of that, at a distance it tends to have trouble breaking on targets. however on the upside you get a lot better air efficiency and can also hold alot more paint in the same size of loader as a standard .68. my old site contemplated changing to .50 but one thing i noticed is that the paint seems to be affected by the wind alot more. i don't know if maybe i was just picking faults and it is no worse than .68 but a colleague and i reached the same conclusion.
there is a great discussion in this section on the cost of .50 and if you check it out it'll explain alot for comparing the price to .68
overall it seems that people have had mixed experiences but generally it is agreed that it has trouble breaking on targets at any decent range (i.e. opposite ends of a supair field).