hi all .. my first impressions:
got it about 5 days back, been busy with it, and am very happy with it. No dead-pixel/reboot problems. Big Thanks to nokia for giving it a very nice hardware.
As someone mentioned, charging cable and headphones plugged in at the same time makes typing a bit difficult, but I somehow learned to use it without a problem.
Loved the app manager, and being able to browse the nokia repositories with it, found lots of apps, its fun to download and use them.
Calendar is awesome ! Thanks nokia and the devs who worked on it!
One thing: kinetic scrolling is good, but sometimes i do feel a need to have a solid scrol bar that I can touch and drag because sometimes some item in the list gets clicked unintentionally and it is sometimes annoying to go back and display the list again and than again start the scrolling by touch.
Camera is gud, but in the start I just was just not pressing the capture button half way to give it time to focus, leading me to think "hey the quality sucks whats wrong with this 5 mp thing", than found out "oops, I didnt know how the focus thing works", and now pretty happy with the camera ... and yea i'm new to using cameras
Stock heaphones that came with n900: Are they the worst on the planet or I dont know how to use them too (just like I didnt know how to use the camera capture button) ? Because read someone saying he foudn them to be the best compared to anything else he tried. Thats not what my experience is with them. I have my own Sennheiser PX100 and they are working excellent with n900, so they r my primary ones. Will donate the default headphones to someone.
Keyboard: I havnt used any mobile keyboard device in the past many years, so I am not used to some functionality that I would want on the keyboard, the idea to me is fresh and I'm happy with n900's kb. Although my friend has n97 and I have used it 2 or 3 times, and I'm glad the keys on the n900 r better. I know n97 has a d-pad, have never used it so I dont know what conveniences the d-pad brings.
It was pretty strange when I found out that not being able to check my balance through *135# is a bug hehehehe, one friend laughed at me, I'm not telling anyone about that again
Battery!!!!!!!!!!!! :::: ok this is the ONLY problem that I feel is a problem for me. I think my n900 is not giving me more time than my laptop with its 9 cell battery, ie max 4 hours I guess. In my experience, a continous use of device, which means that the whole time wifi is connected, browsing constantly (4 or 5 windows open max), downloading stuff, chatting (pidgin also logged in other than the default gmail client), terminal open, notes open, headphones plugged in and sometimes listening to songs, bluetooth OFF, GPS OFF, FM transmitter off, etc, and in 2 hours its like from full to less than 30% or 20% I guess. Its an inconvenience for sure, battery should have worked better. (editing: just found out a post related to bad battery performance due to too much switching between 2.5g and 3g http://talk.maemo.org/showthread.php?t=38380&page=2 ... and since my cell switches between 2.5g and 3g so going to try this and update the post.)
Performance: Let me say this: On one hand, I'm simply amazed at what this device's response time is, compared to any other cell phones I have used, and specially the fact that it feels like a computer and not a mobile device with the level of multitasking. On the other hand, I havnt found it to be perfect like I sense from some ppl's comments here. For example, while listening to songs on the headphones, the sound often lagged when I have 4 or 5 windows of the browser opened (and few other apps ofcourse), specially browser version of gmail, and I had to close the media player because I just thought the media player is slowing down the browser or vice versa, I cannot listen to songs when due to something the sound is just gone for a second or two and comes back, its not a nice experience that way.
Lock Unlock button: very very very nice to have it its perfect.
Screen Shots: its soooo easy to convince a friend to buy this device hehe, while chatting to them I just take few screen shots and email them, and they r like "damn awesome, I'm buying this". One friend was cursing his xbox that he recently purchased cuz now he dont have funds to buy n900.
4 desktops: lovely idea, keep the innovations coming Nokia!
Maemo Community: loving it
The idea of having so many apps as free and open source software has unlimited potential!
Screen: pretty awesome screen, and thanks to the designers for keeping the screen free of any buttons, thats how I imagine a tablet's screen to be. Its beautiful.

(have i used too many exclamation marks and smilies? )

Nokia is an amazing company, the best company to work for .. Nokia has clearly taken the mobile market to the next level.
Love linux!