The positives: light in weight, well sewn corner tabs with proper brass grommets to seat the tent poles, properly sized for the tent's footprint. The tabs also have stake loops, to facilitate using it with the fly and poles alone as a quick and lightweight shelter alternative to pitching the whole tent.
The negative: As shown in the product photo, there is a major seam crossing this footprint. Okay, I can live with that. But it is not taped! taping is a waterproof membrane strip which is bonded over a seam to prevent water from wicking through the thread and stitching holes. Since one of the purposes of a footprint is to deter groundwater from coming up through the tent floor. Though the tent floor is theoretically waterproof itself, that waterproofing treatment is on the interior of the tent, and can get damaged over time through abrasion. You don't want a lake forming between the tent and the footprint. There are aftermarket taping solutions available to remedy the problem, but they cost a bit, and are something of a pain. They are sticky as they dry, so you don't want to do it outside, and they are stinky, so you don't want to use them inside. At this price point, factory taping of that seam, as well as the corners, is a reasonable expectation.
I use this footprint with a Big Agnes Sheep Mountain 3 tent, which has the same tent base dimensions as the Blacktail 3 tent, so I assume that the fit will be the same. It is properly sized to barely disappear under the tent. You don't want a protruding edge: water hitting the tent body would be collected by that edge and guided under the tent. Some diligent types who camp in wet climates place a 3/16" or 1/4" cord just inside the perimeter of the footprint to usher flowing groundwater rivelets of water under the footprint. And keep in mind that a footprint is not a substitute for the careful removal of sharp stones and sticks before you pitch your tent!
This review was written in the old system and had content requirements that are different than reviews written today.