Look for a bong with these qualities:
Thick glass walls with no obvious weak points at teh joints
Easy to clean
Stable base- the Erlenmeyer-flask ones like the Mad Scientist are very stable by design
Don't pay extra $$$ for a fancy name etched into it when a generic but well constructed bong is a better value... Unless you're a 'ZOMG do you like my Gucci bag?!!1!!' kinda toker