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If you're looking for the latest Android device, it's not a bad purchase, but as Matt Miller previously mentioned, there are better spec'd devices on the market, that even take advantage of T-Mobile's HSPA+ network.
The other day I fired up my Nexus One. I have to admit that even though I love my HTC EVO and the HTC-ized Android experience, after having played again, it's clear that Google's out-of-box Android is pretty solid. So, you really can't go wrong with picking one up, not to mention that Google usually upgrades its own devices first.
Are you buying a Nexus S? If so, why?