Hi anjin, For Calendar, reminders and contacts (PIM), and synching with outlook/exchange functionality is very limited, although you can synch with Google Calendar using GPE. Also you can't edit office documents right now, although the ability to edit .doc files is coming soon with Abiword. If this is the main purpose then buying it might not be a good idea.
Reading Ebooks works great with the FBreader app. Be sure to check if your ebook formats are supported.
For mp3, movies and streaming quite a number of applications, though you generally need to convert movies to be playable smoothly.
Browsing and flash work quite well, as wel as RSS, IM and mail (if you install the 'Modest' program. The default email client is not so great.)
I have no idea about usenet software. I don't have experience with the hardware keyboard since I have a n800, not a n810. But for some things like clicking on links, or scrolling in some applications you still need the stylus. You don't need to know much about linux to use this but for some things, like installing beta versions of software you need to use the commandline.
For the money the n810 offer quite a lot of value, especially if you install applications and follow tips on the forum and the wiki to get more functionality, like using a USB keyboard. There is no official news about the successor. Since the previous versions were released roughly a year apart I guess it will take at least half a year.