I was origionally planing to go with the eclipse avn 6600 but then the pioneer avic-z1 came out and i was blown away by all the fatures. Dont get me wrong the avn 6600 is a top of the line nav but there are just some features like xm trafic which HummerNewbie already said, and it works with both xm and sirius satelite radios. Plus it has a hard drive in it which you can store up to 10 GB of music on, although it is a little weird the way its made to upload the sonds. The other 20 GB of the hard drive are dedicated to storing map and nav data. Also you can upgrade the system by adding options such as bluetooth to work with your phone, a tv tuner, an ipod interface and lots of other crap which may or may not be useful to you. The avic-z1 also has voice recognition and as i said earlier lots of other crap. The eclipse may see a little better because of the price, it did to me and it still kinda does seem a better value or as good to the z1, but the z1 has many more features.
I got the z1 for $1700 even though its msrp is $2250, you can find it at many places for alot less than the msrp. The avn 6600 retails for about $2000 but i am sure you can find this alot cheaper too.
I still havent installed mine still gathering up all the stuff im gona need, its my first nav/stereo install in a car.
Sorry bout the essay that i just wrote, but i am very passionate towards this topic.