I use an undergravel filter with my normal filter, it has its pros along with its cons.
First of all, it is NOT a replacement for a regular media filter. Its for the gravel. It works with bubble stones to draw up the water from the very bottom of the tank and under the gravel and through its own filters (usually carbon). Its designed to keep ammonia and/or bacterial build up from occuring in the gravel.
It is usually installed under the gravel against the bare bottom of the tank, mine does 2-3 tubes that the air stones go into topped with the filter holders. I've never had a problem with fish taking any sort of damage in the past 5 years I've owned one, but that's just me.
cons: -It does not filter the entire tank by itself
- Will not pull all fish poo and uneaten food out of the gravel so you still should vaccuum your gravel.
- Not designed to work well with plants
- Dependant on design, it can be hard to clean.
Pros: - stops ammonia build up in the gravel
- stops bacteria build up in gravel
- does help against algae
It is older technology, but with a decent media filter to pick up the slack I find them helpful.