It depends on the plastic...
Originally Posted by dan_fein
Superglue is a very brittle acrylic-based glue that isn't always the best choice for plastics due to its tendency to off-gas while it cures, leaving a hazy residue on the surfaces around the crack. It also doesn't perform too well around water and solvents. But it's very convenient when you have an emergency.
So IMHO, the best choice is an epoxy resin, and there's dozens of different formulations, some of which are more clear and flexible, which is what you'd want for the phone. Some come in a syringe, others in a toothpaste tube, and yet another type comes as a putty, but all come in two parts that you have to mix before applying. So anyway, i've also had great success with a grey and somewhat "grainy/powdery" variety (JB Weld), having used it to fix a broken light switch and to fabricate a replacement for crumbled-out part in a faucet handle. JB Weld can be cut and carved like wood, and that's kind of what i mean by "grainy", it has some kind of a filler mixed into the hardener. The clear varieties look and act more like polycarbonate resin once they are cured.
So if you are satisfied with the way the repair looks, great. If it breaks along the same crack and you want to remove the glue, DO NOT use straight acetone. Mix maybe half a teaspoonful of nail polish remover to 6 ounces of hot water in a jar, and soak for a few hours or overnight. This will make the glue come off without liquefying the plastic. Then just rub the glue bits with your fingertips and it should all crumble off. Dry overnight and then glue with epoxy.
P.S. Please be careful with acetone, even in its dilute form. The concentration above has worked for me, but it might be a little "off" for somebody else.