Okay, once again, my take on x64.
I have been running x64 for damn near 2 months now. I've yet to find anything about it which makes me reget switching to it, other than a lack of Daemon Tools or anything similar for x64. All of my programs run great on it, some a little better than normal. I haven't found anything that I run which seems to be adversely affected by x64 except for Mozilla, and that's only due to a bizarre interaction between NoScript and 64-bit Java. The one thing I do lament is that there aren't nearly enough 64-bit patches for/versions of programs out there to use, and I've yet to figure out how to compile them on my own from source. (According to MSDN it requires running Visual Studio from the 64-bit compiler environment, both of which I have, but I've yet to find a nice guide that walks you through it once)
For the few programs that have 64-bit versions available -- FarCry, 7-zip, a few others -- the performance boost is noticable and significant. FarCry... good lord. Good f%$#ing lord is all I have to say. You thought that game was eye candy before? (Incidentally, the 64-bit patch can be downloaded for free from the FarCry website) Also something which I didn't initially pick up on, the network code for x64 uses 2k3's code as a base, not XP. While I can't site specifics, I had noticed that x64's network interaction seemed a bit more stable and quick.
The biggest problem with x64 -- besides the lack of Daemon Tools -- is the driver issue, namely how many companies don't have 64-bit versions of their drivers available. My Gigafast HomePlug adaptors and TV tuner card aren't getting any use for this reason. That aside, the big companies -- nVidia, ATI, to an extent Creative, etc. -- have 64-bit drivers for their hardware. My x800 Pro is quite happy, and all my onboard stuff (nForce) works great.
Also, as DRavisher and others mentioned, Starforce protection on older games doesn't work with x64 at all, and neither Starforce nor the game developers seem at all motivated to make patches to address the issue. (Both point the finger at the other as far as who's responsibility it is) What this means is that running x64 you cannot legally play any older Starforce-enabled titles; currently the only option is to use a hack to disable the protection.
But there's like, what, 2 good Starforce-enabled games out there that are affected by this?
Anyway, I would say that barring an all-Starforce game library or an overwhelming amount of offbrand hardware in your rig that x64 is worth running. While it's sort of fringe right now, that won't last. There will be more and more 64-bit drivers, 64-bit programs and other goodies for x64 and the forthcoming Vista x64 as time passes. Might as well get with the program now.