They are too isolating to guinea pigs, who love to interact with their surroundings. The urine can build up quickly, as can heat, both of which are hazardous to your guinea pig. Many experienced guinea pig owners will argue that aquariums are fine.
Designer cages
This type of plastic cage is inhumane; it is too enclosed, and heat and ammonia could build up too quickly. I wouldn't recommend this type of cage at all!
Metal cages
The same points for plastic cages apply to metal ones: solid bottom, sides high enough to keep the litter in, and spacious enough for the guinea pig to be active. Beware of sharp metal edges, and make sure that the cage is rust-free.
C&C Cage
Features information and instructions on building a habitat out of cubes/grids and Coroplast, a corrugated plastic commonly used for signs. There are many various options. Definitely worth a look if you're planning to build your own!