I am extremely pleased with the quality of the ALG rails, and while it might seem like they are on the lower end of the spectrum from a cost perspective, I have found them to be feature-packed for their price point. For reference, I have the 8" keymod rail, which at the time was the only keymod option they had and my comments will reflect that.
As far as the rail itself is concerned, there is some machining done to the exterior that seems to do well in aiding grip and I've had no issue with holding on during fire and around the house. To that end, machining on the rail is top notch; I didn't observe any flash or other cosmetic flaws that I've seen other rails at this price point; definitely a plus in my book. In a unique fashion, my forend didn't come with any rail whatsoever at 12 o'clock; this is a standard feature for ALG rails as I'm sure you've noticed. Other models do come with a special piece intended for co-witnessing; mine did not, I just put in for one when I bought everything. Could you use a 'standard' rail? I'm unable to comment on that since I didn't buy one.
As can be imagined, the rail does require a proprietary barrel nut and it does have to be timed, but ALG supplies the majority of the parts required for this with a standard purchase. You will need a pipe for torque; the directions call for a specific type- I don't remember which exactly right now. That being said, it wasn't expensive. I do recommend double checking the shims for their actual sizes since my were not even close to what the color coding suggested, but a micrometer solved that issue. Installation itself wasn't all that difficult after solving the shim problem, and even though you are required to do some "Kentucky windage" in regards to torque, the wrench they supply is designed to work around that issue specifically. Lockup so far is accomplished with six good quality screws and they slot in without issue and the rail hasn't so much as budged. I am very happy with their mounting system, especially after dealing with a certain other manufacturer's rail in the same price range.
As far as weight, I will say that it makes for a great cost-weight ratio, and should be about the lightest thing you can buy before a BCM KMR. Is it better than the BCM?I suppose that is for the end-user to decide, and I do maintain a preference towards the BCM rail where possible, but ALG's offering is also under half the cost and still a solid offering nonetheless. My preference is not a slight against ALG, per se, I own two rails from both makers, and find it very easy to put both of them on the same scale; I just happen to desire the extra weight savings that BCM offers.
If you can't be bothered to read the first half; I think they're a worthy purchase and the sale doesn't hurt either.