Before the FN SCAR hit the major airways, my unit/battalion of the 75th Ranger Regiment performed testing on these rifles while deployed to Afghanistan.
The SCAR is an acronym for, Special Operations Forces Combat Assault Rifle.  The rifle is categorized into two families, the SCAR-L/light version and the SCAR-H/heavy version.  The SCAR-L is chambered in the 5.56×45 mm while the SCAR-H is chambered in the 7.62×51 mm and are both available in the long barrel and Close Quarters Battle variants.  The MK-20 SCAR is the “sniper” variant and holds the 7.62 cartridge.
My unit had a few of these rifles in all variants, the CQB, sniper, and long barrel, while conducting combat operations in Afghanistan around 2007-2009.  After being so used to using the M-4 rifle on a multitude of deployments to Iraq and Afghanistan, we were somewhat skeptical at the new design.  The rifle set itself apart from anything else we had been used to shooting, especially with the charging handle on the side opposed with it being on the top rear.
The Mk 16 Mod 0 was intended to replace the M4A1 rifles that we used while the Mk 18 CQB and the Mk 12 SPR (5.56) currently in SOCOM service, before SOCOM decided to cancel the order for the Mk 16 Mod 0. The Mk 17 Mod 0 will replace the M14 and Mk 11 sniper rifles. However the weapon will only supplement other weapons while issuing remains at the operators decision.
(MK-16 SCAR below)
(MK 17 SCAR below)
(MK 20 Mod O below)
My Experience/Opinion
I personally favored the MK 17 and MK 20 models of the SCAR, the line guys really did not use the 5.56 model and stuck primarily to their M4 carbines.  Not because the rifles were chambered in the large 7.62, but because of its versatility in combat.  With the SR-25 that I carried as my primary sniper rifle, clearing room, climbing structures, etc. became somewhat of an irritating task.  With the SCAR heavies, not only were they extremely light weight, but having the ability to fold the stock made it easier to carry in a slung position.  Another thing that I really liked about the SCAR heavies were the fact that they were extremely accurate!  The SR-25 semi auto sniper rifles that we carried, on average, was a 1 MOA rifle (holding a 1 inch group of 5 rounds at 100 yards).  The MK-17 and 20 SCAR rifles held less than the 1 MOA group, something that is not often seen with the semi auto sniper rifles that I’ve seen. The only problem that I had with the SCAR heavy was the slight rattle noise near the folding stock, it makes quiet covert ops an issue, but with a bit of 100 mph tape, it solves the problem.
Conclusion
The FN SCAR The US Special Operations Command has currently cancelled their purchase of the Mk 16 SCAR-L and are planning to remove the rifle from their inventory by 2013. However, they plan to purchase 5.56 mm conversion kits for the Mk 17, supplanting the loss of the Mk 16.  The SCAR is now one of the competing weapons in the Individual Carbine competition which aims to find a replacement for the M4 Carbine.
