I have a T4-18, it has the pneumatic blow back from G&G. It basically adds noise and some visible motion of the bolt on firing, that is it. There is no recoil effect from the blow back, and the noise is metallic but its quite high pitched, so while distinctive and notably better compared to people without it (in terms of 'realism') its still not a very gun like noise.
My Tar 21 on the other hand has a physical blow back, and its done with a decent chunk of metal. Its loud, and its a more appropriate noise. It does produce a little recoil effect due to the weight of the piece of metal but its mostly about the noise and the movement of the bolt visually (which you'll like look at in game).
On the G&G guns is pneumatic, that means it uses air from the cylinder to power it. This is a pretty reliable system of blow back and combined with the light fake bolt its likely to last a decent amount of time. It doesn't impact rate of fire or air flow and hence it ends up being a reasonable solution. My Tar 21 however is physically connected to the gear box, the bolt is pulled back by the piston and it physically slams back and forth across the top of the gear box with each shot. This is showing some wear after 10k shots and I wouldn't be surprised to find it breaks in the next 10k-20k shots. Its most likely going to snap one of the screws that holds the piece on, they get a lot of wear under the head. But then its also louder and generally better than the what the G&G does in terms of actual blow back effect.
