There are, indeed, some Nokia "haters" (or at least disillusioned & disgruntled users) sighted on these forums from time to time. But what follows is as honest a report as I know how to give.

From what I've heard, the N800 is inferior to the iStuff for video playback if you stick to stuff the iStuff can play. So it's a tradeoff -- play everything, kinda OK, or play only h.264 really well.

If video's a really major use for you, and you're not willing to transcode whatever video you get to a suitable format, you probably shouldn't get either. I've heard some Archos devices are much better. The screen does beat the iStuff for image quality and viewing angle. It's great for viewing photos, but video playback is said to be a bit anemic.

For audio, go N8x0. It's no worse sound quality, handles a wide range of codecs, and has built-in speakers to boot.

Either audio or video really requires a different media player -- I believe we've attained consensus on these forums that the built-in one is no use.

Web-browsing, the N8x0 wins hands-down. Full-size screen means you get the whole web, can read PDFs, everything. Decent browser, though not as good as Opera on the old OS.
AJAX stuff like Google Docs works well, too.

You mentioned whether it has flash? For a moment, I thought you meant flash rom. You mean Shockwave Flash? Dead right! N8x0 has the whole web, including flash games, iStuff hasn't!

Games, I don't know how the iStuff will be when the SDK comes out, but the N8x0 is quite good. DOS emulation, several-other-things emulation (Game boy advance & I-don't-know-what-all), native Linux game ports (Powermanga!) and of course, Doom, Quake, and Quake II. And Flash games (see above) which the iStuff may never have.