I just ordered the last group of parts for my 7.5" .300 Blackout build (bolt carrier group stuff). It's starting out life as a pistol but will turn into an SBR soon enough. I wanted something for close to medium range (0 to 200 yards but capable of 250-300 minute of Ivan in a pinch) with good capacity, larger/heavier bullets than 5.56/.223, and is suppressor friendly
It fit the bill but also came with other positives. Since all you need is a new barrel to convert an existing rifle, everything else would be a standard AR15 part (including the magazine). It's well suited for a short barrel since it's not dependent on velocity (which makes it a great choice for subsonic suppression) and you don't get a blast of unburned powder (since the powder typically burns in the first 8 or so inches)
Factory ammo is expensive, but that's where reloading comes in. I can roll plinking/training rounds for about 20 cents a round (add roughly 10 cents a round if you have to buy your own 5.56 brass to modify). I spent $30 on a saw/jig and can quickly convert 5.56 brass over to .300 BLK brass. I already had a lot of brass from when I used to shoot 5.56, and I have a friend who gives me all of his used brass. Even if I only used each piece once, I probably wouldn't need to buy any brass for a decade
If you get one and use it the way it was intended to be used, I think you'll be really happy. If you get one expecting it to do things it wasn't meant to (i.e. keeping up with a .308 at 500+ yards or matching a 12ga 00 buck round per round at point blank), you will be unhappy