So... in general I'm very excited about this device. It's very new to me. I've used several different mobile phones (both Nokia and not) and my last mobile phone is a Nokia E71.
However, Maemo 5 in N900 is a really different gem than Symbian S60. It's as wonderful as it's different.
My first tip is: Update your N900 firmware immediately!
Updating N900 firmware/software is very easy. Connect to a Wifi network and it will search for updates, and can apply updates OTA (Over-The-Air). You don't need Nokia PC Suite or Nokia Software Updater to do this (under normal conditions).
Stock Nokia N900 firmware had a nasty bug that bricks your N900 after 17 times of crash-reboots. The workaround is simple: on next successfully boot, shutdown N900 using the power button and turn it back on again. The latest firmware solved this problem. (I'm now on firmware 3.2010.02-8)
I mentioned crashes, and yes my N900 crashed a lot. During my almost two days of use, the boot counter is 44 which means it has crash-rebooted about 14 times! Some people also experience N900 crashing but some say that their N900 is just fine. I hope the N900 team will continue improving the firmware so that there will be very minimal crashes.