We use Transcore for all our stuff like this, we service the airports in state as well as multiple garages and toll booths, including the casinos.
They work well, however they require a premade cable with an AMP connector, thread on, which I think is something like a 25 pair (memory). They also are very power hungry, requiring power be located near them. When they start picking up a tag, they're also somewhat loud, making lots of clicking sounds as they start their "dance" when a tag comes in range.
They're not really asthetically pleasing and need to be installed high up to really work properly. Also, any car that has a metalzed windshield, for either tinting or radio antenna purposes (IE: Fords are notorious) they don't work as well as most would intend...passable at best I'd say.