You will find the N900 to be a superior writing device for note taking. I use it to take the minutes of meetings at work. I usually have the minutes e-mailed before the bulk of the people attending the meeting have left the room.
Browsing is excellent. Far superior to any other device I've used. With the addition of the Flash ID faking application Flash works unless the site calls a function that is exclusively Flash 10.x. Flash 9.4 ships on the device.
The map program has many detractors because it lacks the voice navigation most Nokia smartphone users have learned to take for granted. For locating an address it works fine.
You can bump the processor speed up if you feel you need more speed. I run mine at 1100 MHz (1.1 GHz) maximum speed and have no issues with random reboots. Some devices are reported to not support that much over-clocking. Mine, for example, will not tolerate 1.15 GHz as a maximum.
Image viewing, movie watching are all acceptable on the N900. I can not speak to the Mac issues. Someone else who has experience there will be able to help. In the last month I purchased a second N900 to ensure I had one in reserve. I feel it is that valuable for the things I do with it.