Here is my take on it, from my fairly limited experience...
It can grow in very low light (I was keeping it in a 5.5 gallon tank with 8W fluorescent lighting) but it will be very "leggy" - meaning that the internodes will be long. This makes it unattractive, at least in my opinion. If you don't keep it trimmed, it'll very quickly get overgrown and parts of the plant will get blocked of any light. Needles will start falling off, littering the floor of the tank and it'll be an unsightly mess.
If you make sure that all parts of the plant are getting exposed to moderate light (at least), it should stay "bushy" and nice-looking. Hornwort is really good for breeding tanks. Fry can hide in the dense, needle-y growth and I'm sure it's a great medium for infusoria.
I can't remember if it was hornwort or anacharis, but one of them secretes some sort of antibiotic that *may* aid in the prevention of BGA outbreaks (as well as other bacterial infections). And of course, the speed at which this plant grows makes it a pretty good "algae-buster" plant.