Here's my issues with it, and I'll list them in order of importance.
A. This system is designed as a disposable beer dispensing device. Sorry, but there's no escaping this one. The tap is secured to the to the bottle with a little plastic seal, very much the same as a 2-liter soda cap. It's not intended for multiple uses, and it will leak over time. The plastic bottle itself is VERY thin, and wrinkles easily. The tap just feels very breakable. Most of these concerns are addressed with the actual tap-a-draft system, which costs more money than the Coors/Miller bottle. Read: Disposable.
B. The C02 cartridge has a non-standard collar size. You need to use an o-ring, or some electric tape to build up the diameter of the c02 cart. Some people have had luck without doing this, but others have had the c02 leak in less than 24 hours.
C. It has a fixed regulator, and that regulator is set to Light American Lager C02 volumes. This device was made to hold the carbonation of a Miller Light or Coors Light. If the beer i'm intending to serve isn't that carbonated, guess what: After a few days in the C/M home draft, it will be.
D. The label's don't remove easily. I know this isn't a problem for many, but I find it tacky when there's a commercial label on home brewed beer. It just doesn't sit right with me.
Because of those reason I don't find them to be a good product to use full time. My Keezer setup was under $350 total, and that was with a brand new Freezer. For me it was just worth having equipment that will last a lifetime.
With that said, I do use the Coors Miller systems. I find them great as portable/disposable vessels to take beer to a BBQ or party. I can quickly fill it from my Keezer, and dispense 1.5Gal at another location. I usually just throw it away when I'm done with it, as I have friends that drink BMC, and it's pretty easy for me to get another one. I have to say, they are absolutely fantastic as a growler replacement.