The SIG 552 was my first, and 3 guns later, it's still the only one I field. I am the third owner and it still shoots true with its original hop-up. Being a guy who likes to shoot... alot (20,000 rounds or so since August), I've stripped and rebuilt the internals from the ground up to make it one of the most reliable outdoor/cqb guns out there that still kicks like a mule. Right now, here is where the scores lie for the various criterion that has been laid out:
Solidness/Feel: 8 out of 10
The foregrip, body, rearstock, and handgrip are all plastic on the TM SIG552. Metal parts include: entire front sighting assembly, rear sights, and alot of structural bracketry that holds body panels together. Due to the construction of the "infrastructure" of the gun, it still remains very solid and has minimal creaking. TM designed the body so that pretty much every plastic body part is either connected to, or sandwiched between pieces of metal. There is a full metal skeleton of the body in which you more or less, plate in plastic as you reassemble the gun. I have splurged for some metal accessories but unless your hellbent on dropping some serious coin the metal body, although really sweet, just doesn't justify its cost.
Reliability/Durability: 8.5 out of 10
This gun utilizes the TM Version 3 gearbox which is more reliable than Version 2's, especially in colder temperatres. I have also heard that they accent internals upgrades better than Version 2's. The hop-up mechanism doesn't use any rubber hop-up bucking; when you roll the mag in, the BB's line up and seat flush into the bottom of the hop-up housing. I have mentioned before that this gun has never had its hop-up changed, and still shoots true with regular maintenance. I don't know if the lack of hop-up a rubber bucking was a specific design point, but it does save you from replacing another part regulary.
Flexibility: 9 out of 10
Here is where the design of the SIG really shines. Apart from having the side folding stock (which is solid as hell by the way), the SIG also features, a completely ambidextrous fire selector; and coolest but not least, a trigger guard that flips and locks to either side so that you can use it while wearing winter gloves without getting the material caught between the trigger and trigger guard. (Real steel design) Add to that the bottom RIS rail that comes standard for a vert grip, and the option of getting additional side rails for the handguard, you can put together one fine accessory mule of a gun if you were so inclined to.
Upgradability: 7 out of 10
Alot of people are saying that the SIG (552 model especially) is a battery whore, meaning that it is nearly impossible to fit higher voltage batteries needed for an upgraded AEG. Through research and DIY work I have found this to be false. The SIG handguard is a cramped space designed to fit a 600mah 8.4v battery, this just won't do. By dremmeling out some of the baffeling from the inside of the handguard I was able to fit a 8.4v, 1300mah battery in; which is fine for a stock platform.
I personally am running a 9.6v 2300mah nunchuck pack, I keep one half of the pack inside the foregrip, then wired the second half to the outside of the hand guard and hockey taped everything in place. This allows me the 9.6v needed for the M100 spring, and enough mah so I don't have to worry about mid-day battery issues. Of course you could always use a battery bag, or AN/PEQ box, but my nunchuck battery ran me less than $25 with shipping to my door, and cost me 50 cents worth of hockey tape to black out the pack and attach it as needed.
One of the down sides of the design is the shorter barrel length of 247mm, so it's more difficult to get the FPS up to where you'd like it for an outdoors gun. With the addition of a standard length 155mm silencer, you can easily fit an M4 length 363mm innerbarrel, which falls about 3-3.5 cms short from the end of the silencer to ensure that dirt doesn't sneak its way up there.
Overall: If you like the unique look of the SIG 552, do not allow minor details to ruin the plan. The body and feel is solid, (through my experience) it is extremely durable, the gearbox can easily accept upgrades, and the design is meant to be flexible. That being said, one cannot ignore the issues with the battery space, and barrel length for higher FPS applications; but the battery capacity can be dealt with easily, and with the addition of a barrel entension, the FPS challange can be easily solved.
Obviously I hold a positively bias view of the product, but then again, I have good reason to. Excuse me, I'm going to go shoot some shit.
- Ryan