Yes, we added the Garmin 600 Send and Receive unit. You can see photos of it in the Photo Gallery -- look in the Tugnuts Owner's Section Photos under "Circle T" and you'll see how we installed it and photos of what it does for us.
When we installed it about 2 years ago, there were no "receive only" AIS units available (that I knew about anyway). As you'll see, the 600 requires a dedicated GPS be installed, where a receive-only system does not. I would choose the 600 again as I like being seen, but would encourage you to at least get a receive-only system so you can see others. In my opinion, once anyone operates with an integrated AIS display on their main Garmin navigation display, they won't want to leave their marina without it.
The first time you see that a ferry is fast approaching from the other side of the same island you're approaching, or you get a warning alarm because a fast moving ferry is overtaking you from the rear and will be on top of you, but not for 6 minutes (or whatever time you set) -- allowing you to knowledgeably make a small turn now to avoid a big surprise later --you'll forget about the cost.