My Gemtech M4-96D on my M16 makes it a whole new gun.
I have it on a factory Colt 11.5" M4 Commando upper and I will never fire it again without the suppressor.
First of all without the can, it is as loud as a 300 Win Mag. With the can, you can fire it without hearing protection. Very little (just mechanical) sound at the gun and a sonic crack out in the distance. The whole thing is the sound of a 22LR.
Second, it makes the gun VERY controllable. You would swear that it was a 9MM (well almost). Two/three round bursts are real easy and longer ones are a pleasure. Heck had my buddies 11-year old son firing it (just loaded with 4 rounds) and he had no problems at all. Heck, just wanted to fire it more.
One draw back is something called "suppressor blowback". Firing an M16 (don't know about others) causes gases to be blown back throught the ejection port and requires goggles. Also, throws the empties forward instead of to the rear. I've done some research over on subguns.com, and this seems to be normal. Not at all hard to get used to.
I don't know about on a semi-auto, but (for me) in full-auto with a shorty set-up, it's a must.
I have a Gemtech Talon-SD suppressed 9MM upper coming. From what I've seem with the M4-96D, I'm really looking forward to this with 147gr subsonic ammo. Should be quiet as hell.
Getting a suppressor for your MG really puts the iceing on the cake. You can really concentrate on firing the gun and forget the confusion of the unnecessary noise and recoil.
Hope this helps.