I've a got a Seatec vest too: the 3 kilograms camo type.
Feedback: very positive.
It fits like a glove on my upper back, and the weight is so well balanced and well distributed that I don't even slightly feel any weight at all. (the weight is made of flat metal plates, inserted into the neoprene vest and anatomically distributed on the upper back).
If we had to necessarily find something wrong with the Seatec vest (but we don't have to: it's an excellent piece of kit indeed), there are only two possible downsides: the weight plates are stitched into the vest, so you can't take them away or add more weight, unless you break the stitchings from the neoprene. Anyway, this is not a big downside: once you know it's 3 kilograms, keep it 3 kilograms, why modifying for the sake of modyfing? Just keep it as good as it comes.
One second feature that you might consider a downside: it has no "ball" for securing to the belt, but a clip. So that, if you need to drop your belt in case of danger, first you have to unlock the vest clip. But on the other end, unlocking the clip is easy and quick, just a moment, so I can't see this as a real problem.
Seatec vest: very positive feedback from Spags.