I'll try to remember to copy some of the details down and do a little more testing on some of the things. Off the top of my head;
Rail attachments;
Forwards grip - reduces horizontal recoil. May have some effect on vertical recoil, not sure.
Laser pointer - reduces cone of fire/bullet spread. Mostly has an effect on hip fire, maybe some effect when aimed.
Flashlight - purely cosmetic right now I believe.
Barrel attachment;
Suppressor - reduces sound greatly and muzzle flash somewhat. No declared drawbacks, but may have some undocumented impacts.
Scopes;
... most of these are what they say on the tin. Not sure if the different types have any different effects or are supposed to in the future. Just go with the magnification and aesthetics you want. Oh, plus the IRNV night vision.
Those categories are the three with avaliable accessories that can be equipped at the same time. Pick one of each. Right now both rail and barrel share a selection slot
but still work at the same time. That is, the suppressor works alongside any rail attachment despite being in the same (utility) selection screen. You just have to select the two you want so they are both highlighted. They will show up on the model despite the slot only showing one of them.
Similarly a couple of items seem to replace other things
but actually don't. The grenade bandoleer goes in the same armour slot as your shield but doesn't appear to actually change anything (not tested, but didn't notice any difference during fighting). Similarly the engineer keeps explosives in the same slot as his ammo pack, but the ammo can still be accessed by selecting the turret and choosing alternative fire mode (b).
The foregrip is also either totally worth it, or it's one hell of a placebo.
I'm starting to think the latter. For my Gauss SAW I switched back to the laser pointer and noticed my hipfire working a lot better without losing any ability to keep sustained fire on targets. I want to reinstall FRAPS and do some real comparison tests, but I'm fairly certain the cone of fire is wider than any horizontal recoil and the vertical recoil is controllable without it (if it even makes a difference). Shrinking the CoF is more important anyway in my books.
I know I bought into it immediately after a patch that seemed to improve SAW accuracy anyway and my FPS was improving at the same time, so any actual impacts might have come from those instead.