The Mini-14 is a gas-operated, M1-look alike carbine chambered for 5.56 NATO (even though marked as ".223 Rem.").
Pro's:
Light.
Reasonably rugged.
Uses a unique gas design where the "cylinder" is in the operating rod and the "piston" is a nipple in the gas block. System self-vents to the atmosphere once the operating rod begins to move, which affords some measure of metering (and, therefore, some flexibility of ammo use). Far easier to clean than M1/M14/M1 Carbine. The forward end of the operating rod is not splined to the barrel, but rather is captured by steel inserts in the stock: on the one hand, disassembly is a bit easier; on the other hand, after-market stocks can be problematic.
Cons:
Not very accurate (supposedly Ruger has altered the barrel profile a bit to improve machine accuracy).
Bolt hold-open device is not very secure (bolt will close if you bump the butt plate; very dangerous is there is a loaded magazine in place).
Good magazines are hard to come by. Factory Rugers are expensive and rare in large capacity; after-markets are unreliable.
The M14-type magazine catch system means that most mags will not drop free, which slows down a combat reload.
Factory iron sights are mediocre, and scope mounting is complicated.
Bottom line:
A stainless Mini-14 makes a good short-range plinker, anti-pest gun, or defensive carbine. An M4 clone, however, is more accurate, more customizable, and a better appeal to a rifleman.