I like to look at my SG552 as a "project gun". Out of the box, it is not performance worthy in any sense of the way. There are multiple things that limit it.
1) Mags are expensive, and not common. The midcaps are not cheap to purchase, and the low-caps are even more expensive to purchase.
2) The gearbox internals, aside from the gears, are not very good. The gears can hold their own for a while, I'm personally still using stock gears.
3) The hop-up is not cheap and is rather difficult to find. When you do find it, it costs a pretty penny since only TM makes a decent one (Note that the G3 hop-up is applicable, but requires you to modify your mechbox, which I am now suffering problems with. Do not do it).
4) A long list of other things.
I've written an extensive review that also has gotten a good amount of public rating rating and replies from the general public and their experience with it. Take a look at it. It is still incomplete on the disassembly manual, but that's just a technical problem since this computer is being stingy with getting the photos. Hoping to get a proper disassembly manual fully uploaded this summer, albeit being long overdue. Hoping to get the additional information I have merged into the review once I've incorporated it properly into the review.
Link is here -> http://www.airsoftcanada.com/showthread.php?t=87465
If you are looking to perform adequately outdoors without sacrificing indoor functionality, go with an M4. My brother runs an M4 and performs well in both areas. The M4 uses a 363mm inner barrel, which is long enough to get you decent shots mid-range in a forest-setting, as well as being short enough to maneuver effectively indoors. It's pretty much the cost-effective middle-ground between the M16 and SG552.