The .300Blk is a great concept for a subsonic round, and still performs awesome as a supersonic round.
While it's fun... it's not the most economical for non-reloaders... Especially considering the plummeting prices of .223 and 7.62x39 ammo.
If you're seriously debating an inexpensive plinker, and you want a .30 caliber rifle, consider a 7.62x39 build and ask yourself the following;
Do you reload?
Yes: As stated above, the .300Blk has near unlimited resources available. The brass is plentiful and the .30 cal projectile is available from 90-220 grains.
No: 7.62x39 can still be purchased in bulk for less than $0.25 a round
Do you shoot suppressed?
Yes: The 300Blk subsonic rounds were designed for, and take full advantage of suppressed shooting.
No: You can technically suppress it, but the x39 ammo is dirtier and their subsonic loads are few and far between... usually costing $$$
Is having interchangeable parts a necessity for you?
Yes: The 300Blk uses all the same parts as the standard AR15, except for a barrel swap (of course).
No: The 7.62x39 requires a caliber specific bolt, as well as a caliber specific magazine.
If you just want a 300Blk, get it. You won't regret it.