It's a gift for anyone with a launchpad account
It's not an upgrade nor a downgrade really, as a phone it beats all te devices I've used, quick for stuff like calls and texting.
The N900 may have offered more features but frankly, it sucked as a phone, choking when there's an incoming call.

The UI is well thought out really, best there is right now, it's not a standard looking OS but anyone I've handed the device to could explore it easily, whereas with the N950 I had to explain swiping.

As for what's under the hood, there's not much, multitasking is suspending and resuming the apps, it even says resuming at times even with the 1.4GHz CPU, probably due to an IO bottleneck.

For the average consumer it's a good device all in all, for most Maemo power users, it's probably something you'll not want to get.

BTW, WP7 is a strong comeback really, WM sucked a lot compared to it