Brian Flood has what could possibly be the last update to Arc2Earth before it is released to the public. Brian also has posted the price of the extension ($100 US) so it is going to be very reasonable considering what you can do with it. New in the latest beta builds is improved support for graphics right out of ArcMap. I can see how you can really use this for red-line graphics or quickly highlight areas. While there are other KML export tools for ArcMap (that are mostly free), there are none that even come close to the scope of Arc2Earth. Brian’s got some more screen shots on his blog so you’ll want to click over if you are interested in really leveraging ArcMap and Google Earth together.
The KML/KMZ that Arc2Earth generates will really give ESRI something to test ArcGIS Explorer with, that is for sure.