there are a few different types that i know of. The DIY, the "U" tube setups, and the pump driven setups. I went with the CPR CS90 overflow which is a pump driven overflow and it is awesome IMO
. I don't know much about the DIY and "U" tube system other than the "U" tubes tend to lose siphon a lot from what i've heard. I went with the pump setup from the start.
any overflow will of course lose its siphon once the water gets below the overflow intake, this is why precautions have to be taken to not overfill the sump. you need to allow for the extra water from the system to back up in sump without overflowing.
This is the system i got.
CS Overflow - Overview and Installation - YouTube