As an owner of a N900, and having used an iPhone and Android device once, I find Maemo's UI eye-gougingly horrible.
The UI concepts are great (top-left everywhere to multitask, click the empty space to go back, bottom-right for fullscreen toggle, etc.)
But the execution of the design is terrible. Nokia really doesn't know how to make advanced features easily accessible.
To prove my point: Look at the wireless settings page. It looks like you scaled network-manager to that tiny screen.
Now, look at the default file-browser. THAT'S HOW IT'S DONE, NOKIA. On a tiny device, you do NOT have controls that small.
Next, look at the Phone application. That's supposed to be integrated with the contacts application!
Did I mention how much MicroB sucks (checkerboard loading pattern, nonhildonized addons window) ? Or that ugly ripped off slide to unlock screen?
Man, I wish that MeeGo HE was usable enough for daily use. I love how Maemo is open and all, but on a phone, no matter what you do, it sucks, plain and simple.