I had the 770, two 800's, and two 810's, but currently I use the iPhone. I just had a friend over for Christmas who owns the N900. There is no denying the platform is the most powerful platform available on a mobile device. There is also no denying that maemo has come a long way since it's introduction in 2005. However, I still feel that a few key pieces are missing to make me want to switch to using the N900 over the iPhone. For one, the screen response is terrible. I know it is resistive versus capacitive on devices like the iPhone and the Droid, but do I really have to press so hard to get a list to scroll smoothly on the device? Perhaps if I haven't been an iPhone user for so long I wouldn't notice so much, but that alone makes the device unusable to me in a daily situation, I would get frustrated every time I used the touchscreen.
I am also so over using a stylus on a device at all. I know it's more accurate, but guess what, I'd rather have software which can figure out what I mean by a fat finger press.
While most of the UI of the N900 is pretty impressive, there are still long gaps with nothing displayed on the screen when an application loads, or no response when a button is pressed on the touchscreen. I'm sorry, but for my tastes I'd rather have a device which provides a more seamless experience but requires me to come up with workarounds.
For example, in order to connect to the Linux server I run at home from my iPhone I use an app called iSSH which is pretty similar to putty on windows. However, I have set up key pairs and screenrc so that i press the hostname in the app, the app connects me, and then screen reattaches so I'm left right where I left off, but I don't drain the battery by walking around having an active ssh connection back to my home machine while I'm out and about.
The media application on the N900 has come a really long way though. I run a uPnP server at home on the same Linux machine and there is no way for my iPhone to connect and read the directories with TV shows and Movies available. The N900 on the other hand detects it automatically and has no trouble playing 90% of the content directly through the File Manager app.
At the end of the day though, the N900 just isn't for me. I already have a laptop at home, and I have the iPhone which takes care of all media/connectivity needs that I have. The only thing that is really lacking for my personal use is small icons identifying when there are unread emails waiting, but for now I can live with that limitation.
UPDATE: I forgot to add that although the development for maemo devices is open to anyone with a repo, the number and quality of apps is staggeringly low. It is definitely a niche device, which is really unfortunate because the N900 is hands down the best hardware out there currently, but I feel like it's achille's heel is always going to be the limited application availability.