Just picked up an EoTech 3x magnifier with the FTS (Flip to Side) mount. I'm by no means an expert in this stuff, but I thought i'd offer my first impressions nonetheless.
Optic: It looks great. It's clean, clear, bright, and sturdy. The reticle takes up an appropriate angle in the magnifier. Specs: 3.25 x fixed magnification, 1.65" eye relief, 6 degree field of view, 4.4" length, 6.5 oz weight, -20 to 140 F operating temp, waterproof to 33 ft, sealed fog proof internal optics.
Mount: Pic rail mount part is tight, and light. It attaches via a single screw (flathead *sigh*). It's ambidextrous. To use left handed, you just turn the mount backwards. The optic is mounted to the pic rail mount via a 2nd mount, which is quickly detachable by a small lever. The mount is adjustable both horizontally and vertically. There is room for improvement here. The adjustment screws are flatheads (boooo), and not replacable. There is not a large range of adjustment, either. Fortunately for me, it lined up well enough and wasn't outside the range. Also, the tension is not constant across the adjustment screw range. My concern is that if you are close to one extreme of the adjustment on your weapon, it may be loose and start to wiggle around a bit after firing. Also, the 2nd (upper) mount is connected to the lower pic rail mount on a single, relatively narrow rotating lever . This allows for some play. It shouldn't affect accuracy, but can wiggle the reticle around in the magnifier (probably a few MOA in the field of view). A better design might be a wider lever, or 2 levers (one in front, one in back).
Use with Larue QD mounted EoTech holo sight: It fits well. The magnifier and holo sight are along the same plane (i'm sure there's a fancy name for that that I don't know), and provide full field of view in the optic.
Overall: It's a good optic, with some room for improvement in the mount. It will still function relatively well, but I'd expect a little more from a 500 dollar solution (the 4x magnifier is even more expensive at 800-850 bucks).