Be careful using Superglue, it's susceptible to weakening of the bond in contact with water. Personally, I wouldn't trust it at all. An epoxy (such as Araldite) would be a better bet, but even then, the surface area would be low compared to the bond strength you need. If you have a large cross-sectional area in the break, and are prepared to risk a lap full of hot coffee, try an epoxy, but unless it's mechanically fixed (e,g, staples) I'd buy a new mug.
John
March 2008