I have a 6x45 built on a Mega side charge upper with an Young Manufacturing BCG. I like it a lot. The design is simple and rugged, and you can use the charging handle to take the place of the forward assist should the need arise. After using it for a while I don't think I'll ever own another rear charger. It's not ideal for lefties, but I'm not a lefty.
The cons? You lose the dust cover and the shell deflector are the only ones I can think of. My rifles are target/hunting guns so the dust cover isn't a huge deal and the charging handle sticks out far enough that it should keep the errant shell case or two away from my cheek.
I'm in the process of slowly converting most of my rear chargers to side charging.
I'm in the process of parts gathering to build another and am looking at receiver/carrier options. Young is now selling a side charge upper receiver with a movable cover for the gas key channel. It looks really good, but is pretty expensive by the time you buy the receiver and bcg. JoeBobs has a side charge upper/bcg combo that looks attractive for about $100 less than the complete YM. It also comes in an ambi version for the lefties. Decisions - decisions.