Fully agree with adam and junior, the ICS split gear box is a very good design and very quick to change if you have a spare one already made up with a different spring.
Also to fully up on junior comment about bodies and stocks. Yes and ICS gearbox setup will only fit into ics bodies, also you need to be aware that the ics hop up is the only one that fits. But don't let this put you off as the ics hop up unit is pretty. You can also put other stocks on a ICS, but if you decide to change the buffer tube. Then you will nee to slightly file down the stock adapter.
Also the front ends can be changed, but depending on what type you go for then you will need to slightly modify the end of the barrel. By removing to of the lugs where the delta ring locks to.