My introduction to NVGs was US military issued Gen 3 gear so I'm probably going to be a bit harsh on this.

#1 Beef: The head harness can't be aligned with either eye because there's no horizontal adjustment on the mount. Also gives me a raging headache after 2 hours (might be me).

The Gen 1 tube amplifies twilight best (EENT). The variable front focal length seems good on paper, but there's so little tension that it falls out of focus with every step. The diopter adjustment is the same way. The focal alignment from eye to aperture is extremely sensitive. You have to have it exactly level, both horizontally and vertically, and exactly lined up with your pupil to provide a useful image. Turning on the integrated IR illuminator usefully gets you about 50yds in thick central Texas scrub in moonlight
All in all, I honestly haven't had a chance to use it with my Sightmark SM14002 red dot yet so I may update this when I put it on a stable rifle, but only buy this thing if you plan to sit on the porch at night and watch the opossums walk by (neat if you have kids).

Magnification, resolution, blur, and tube artifacts were all within expected parameters. I hope it'll work a little better on the rail mount.

Sometimes would not turn off. Others it was on when I didn't know. Useless without IR light on. Not much better night vision that the night mode on a cam corder.

first one had spots on screen ..replacement works great

I love this optic. It works extremely well for a gen1 device and does everything I need it to. I think that most of the negative reviews come from people buying this who don't understand what they're buying or have no real experience beyond Call of Duty.

Subject to atmospheric conditions like dust and humidity:
On a moonless night I can identify a human shaped target at about 50-75 yards. On a full moon night I can identify a human shaped target at about 125-150 yards. Note that identify means "see" a human shaped object. You cannot distinguish fine details and legs are usually blurred together at that distance. For applications like hog hunting, when paired with a good laser, this device performs very well. Dark hogs stand out against trees and eyeshine from the integrated illuminator make systematic removal of bacon an easy task.
I can and have (on private property) driven a blacked out vehicle successfully with this device. It is roughly equivalent to looking through a toilet paper tube. This does not mean I recommend trying it yourself, but that it is possible under good conditions.
If you intend to walk with the device, practice is a good idea since it takes some getting used to.
During hazy or dusty conditions the device actually works better without the illuminator turned on since the airborne particulate bounce all the light back into your face prematurely.

Personally, I prefer the monocular to binocular. I find having one eye that I can use for peripheral or close viewing advantageous. Since most nights usually have enough ambient light that you can see at least three feet in front of you, it is nice to be able to close one eye and do technical tasks or limited navigation with your free eye instead of having to continually re-focus the night vision or flip it out of the way.
Be advised that some people do have trouble with only being able to see through one eye. If you have no trouble shooting with both eyes open, or can walk around in the dark with a flashlight holding a tshirt over one eye to obscure that eye's vision then you'll be just fine with this.
By setting the monocular up on my support side I am also able to use weapon-mounted optics with my strong eye without having to fiddle with the night vision.

It is possible to see through scopes with this, but you will have to get the distance and focus just right. Practically speaking, it's easier to just save your money and buy a laser instead of a mount for this optic.

Tactically speaking the illuminator switch can be 'bumped' for a quick flash of illumination without needing to fully turn it on and off. When turned on the source of illumination is visible to the naked eye if you are looking directly into the beam as a very faint red dot a little smaller than a dime. At distance it is actually easy to overlook the small glowing dot. You can see other sources of light, visible and IR. People also generate eyeshine like animals do under IR for some reason.
As for the fisheye effect: some people hate it, and others don't mind. Personally I actually like it a little. When focused properly you can actually use the fisheye to view objects that would otherwise be out of focus for where the center of the lens is focused.
The lens itself actually focuses down very well and I have opened combination locks in total darkness and can read a wristwatch.

All in all, I really enjoy using mine. As someone who can't justify spending +$3000 on night vision, I am very satisfied with this product. It does everything it is supposed to for a gen1 device. Don't buy it thinking it's a GPNVG-18.

Terrible.. save your money or take a home equity loan and get PVS14's.. gave these a chance off of some "they are okay and will do" reviews. They are not okay, its like looking thru a toilet paper tube you cant even walk with these let alone shoot with these on. And as if they weren't already bad enough to see out of they had a huge black imperfection in the lower left (normal and acceptable according to manufacturer). no thanks. Very grateful for amazons return policy. Used the money to buy ammunition instead.