i have an SRC, and i'm pretty happy with it so far. if a plastic lower reciever doesn't bother you, it feels really sturdy. i'll let you know in a bit how easy it is to upgrade, I'll be changing the hopup rubber, adding a sorbo, changing the piston, and inner barrel and air nozzle.
the air nozzle leaks a bit, but the piston head/cylinder head is perfectly air tight. i've heard the piston, and the gear shafts/bearings are poor, but i haven't had a problem with them. also the cylinder in all of them are full seal, which people say is bad for the gear box, again, have't had a problem.
i have not used the stock spring. ROF is OK.