The Nexus One is the only modern Android phone (besides the original Motorola Droid) that runs pure Android- it doesn't have any of the additional software like HTC Sense or Motorola Blur.

It's also usually the first to receive updates, it got Froyo way before any other device.

Pardon my ignorance, but is it possible for someone with a Sense phone, say, to install pure android on it and get all the advantages of the Nexus One, with the updates and all? Or do you need drivers or something?

It requires rooting/jailbreaking your phone and unlocking the bootloader, if you consider that an option. Otherwise, American carriers (except for T-Mobile) will freak out if you even mention running something other than what they put on the phone.

Ah, sure, that's fine (I ran Android on my iPhone once). Does it work perfectly otherwise, or are there any incompatibilities?

You can, but it's not easy and it's definitely not something a normal consumer would do.

Although I imagine the people buying the Nexus Ones from the developer site aren't normal consumers.

There are one-click root apps floating around for the various phones. Combine with ROM Manager or CM Updater or equivalent, and it's basically like the carrier experience.

I would recommend it to anyone who actually cares about their phone.