They are super fun to shoot, but the factory ammo is insanely expensive for what it does.
I have a Kahr/Auto Ordnance M1 Carbine that I've tricked out with a Choate folding stock, replaced the upper handguard with a Picatinny rail, and mounted a SigSaur red dot. When I go to the range, it is the one weapon that I get asked most, "Can I try that?" People are amazed at the lightness, natural pointing ergonomics with the the pistol grip stock, very light recoil and lightning fast recovery time between shots. It is my weapon of choice for coyotes (the four legged variety) in the semi-dense woods around my house where I'll never get a shot over 100 yards.
But you are right - ammo is extremely expensive...but I would not (and do not) hesitate to consider it a more than adequate PDW with Cavitator Ammo - which is even more expensive than the 7.62 I need to feed my AR-10.