This is my favorite Android device currently on the market. Why? Eris is slim, supports pinch--and-zoom Web browsing, and runs on a solid network with fast 3G data.
I'll take Eris over Moto Droid thanks to Droid's lousy hard QWERTY and lack of pinch-to-zoom in the Web browser and photo albums (though the third-party Dolphin browser now offers a multitouch browsing solution for Droid). I don't drive all that much, so I can live without Droid's Google Navigation until Eris gets its Android 2.x update. For more on the Eris-over-Droid line of thought, check out this thread on HowardForums.
And I'll take Eris over Sprint's Hero because it's slimmer and I prefer the all black look - though Sprint's Hero is a better monthly value. Eris also seemed to be a smidge faster than Hero in hands-on tests, though software updates to both devices could change that.
HTC just launched a North American GSM Hero with AT&T-ready 3G banding, but AT&T's performance has been so lousy out here in Northern California that I'd give up that phone's iconic styling in favor of a better performing CDMA carrier.