Our review unit came a little later than we might have liked, but for anyone in the audience still down with an exhaustive hands-on and setup photo gallery with Apple's purdy new AirPort Extreme 802.11n Base Station, we've got you covered. Click on for our impressions, or check out our photo gallery.
So, here's the rundown of what we found: setup was exceedingly easy (obviously), although only having three hard 100Mbps Ethernet ports kind of hurts for a power user's home router. (It also made totally unthrottled bandwidth testing to gig Ethernet impossible, bummer.)
After we ran the install software and the 802.11n updater, even with our Core 2 Duo MacBook Pro sitting basically on top of the Base Station (ok, it was a foot away) we only got about 150Mbps -- not terrible, but not 10x. Sustained transfer didn't exceed 35Mbps even in burst, which was kind of a bummer, but apparently your mileage may vary, since other tests demonstrated a kicking performance increase.
On the other hand, it's certainly the least aesthetically offensive of the 802.11n routers out there, with a flat, compact MIMO antenna array that doesn't look like a soviet satellite installation. Hey, just saying, you take the good and you take the bad.
Update: We fiddled around with some of the settings and did indeed have better luck as we moved to 802.11n-only mode, especially in the 5GHz band. Although we couldn't get that base 802.11n (in b/g/n compatibility mode on 2.4GHz) rate to boost up any higher, we might recommend that you consider which kinds of machines will be living on your network, and whether a .11n-native 5GHz setup would be an appropriate compatibility tradeoff to get up to the full theoretical 300Mbps.
When we were connected on 5GHz at 300Mbps, we saw performance jump from an average of about 30Mbps to between 65-75Mbps sustained; still pretty good, but not pushing the typical cap of 88-92Mbps we get to hard Ethernet-to-Ethernet machines on that network. However, again, YMMV, as radio interference and thus network performance can vary from lab to lab, room to room, and household to household.