Nick79: Yes, they really work...but the design is fairly dated. I have used both the ART I (pictured w/ the boonie-hatted feller) and the later ART II on various M21s over the years. The ART I (as issued during the Viet Nam War and into the late 70's) was pretty reliable.
Both scopes would allow an M-21 equipped sniper to routinely engage man-sized targets out to 750-800 meters. With practice, a good zero, and a good spotter, I could get good hits out to 1000 meters about 7-8 out of 10 attempts on the first shot. The stadia lines were designed to range an average sized soldier's torso and use mil-relation formula to dial in.
The ART II (1980's) was my first experience with Tactical Marketeers taking a good design and making it bigger, longer, heavier, blacker, more tactical, twice as expensive as the original, and less durable. It was the first time I saw a pretty good design get made-over into something REALLY TACTICAL!. The Army liked the ART I and just forked over the cash when the ART II was offered...big mistake. The ART II's fearsome appearance concealed a fragile design. I had two of them break during a sniper school and finished that course using a third (and final) scope. Problems with gas leaks, fogging, reticle completely falling out of mount, etc. Additionally, the ART II's range dial had no scribe line for actually being able to precisely dial in and set a determined range. The whole setup was ridiculous compared to the perfectly fine ART I. Also: neither scope was designed for reliable retainment of zero after detaching from M21 mount (the ART being held to scope base by a single thumb-tightened screw). Each sniper had to swag the tightening of the mounting screw (in the days before issued torque wrenches). This, combined with the crappy range indexing, and the QC problems, caused the SOF community to lose the ART II and replace with Leupolds (which are still used to this day).
For all that, the ART is still a pretty good precision tactical scope, and the Cabelas price is about 1/2 what they used to sell for in the 80's. There are better hunting scopes for game, but I still think it's a pretty good buy for a sniper scope...