I fitted an AIS receiver last year, the Garmin with built in VHF radio. I really didn't get on with the radio so decide to replace it. There is a Garmin receiver with out vhf but I found AIS such a useful tool I decided to replace with a Transmitter/receiver (Garmin AIS 600). The only drawback to AIS currently is that only a small percentage of boats have transmitters, I couldn't moan about it if I didn't have one myself so I joined the club.
The Garmins have a proximity alarm for AIS targets, it's a PITA when in/near harbours as it goes off constantly, can be turned off easily but it switches on every time you turn the plotter on. But it is a safety feature, at least you don't need to worry about manually turning it on. The parameters are adjustable too.
It goes without saying, if everyone went out and bought a receiver without t/x then we'd all be wasting our time and money, there'd be nothing to see.
Obviously it's a legal requirement for anything over 200 grt, personally I hope it goes the same way as DSC and a transmitter becomes law for everyone.
If you can afford it, take the initiative and buy one!