I've switched for a Desire Z, I was tired of the missed calls, the laggyness of the UI, the mail client which is a joke, etc.

Both phones have pros and cons.

I prefer the screen of the Desire Z, the UI is much more responsive and fast than the one in N900, the mail client is very fast, the phone functionalities are better, the UI is polished, there are a lot of apps, the browser is very fast and responsive, etc...

But the N900 has a better keyboard, has more gadgets (IR, FM transmitter etc), is more open, I have the feeling to control the phone in maemo, where in android I have the feeling that a lot of things are not under my control, the external speaker of N900 is better, the task manager is far better (no way to close an app by default in android, no preview of the apps open), etc..

Overall, I would say that android is better when it comes to use the phone only as a phone, or for average joe.
N900 is good, but annoying when it comes to basic functionalities (phone / mail / surf)