Windows 7 x64 here. I miss some of the great x86 utilities like DefenseWall and some very good antirootkits (diagnostic purpose). Now x64 is also vulnerable to rootkit attack.
64 bit is my choice, mostly because of the Ram advantage, and with windows 7 there has become a good integration whit 32 bit programs. But sometimes I can have a a hard time finding 64 bit drivers,....... driving me so nuts that I wish me back to a 32 bit version. But you can get it all "Yet"
Can't tell the difference, except when using demanding applications 64 bit is faster. I can have either system installed within 8 minutes, although I use Vista 64 bit most of the time.
All the new Windows 7 machines I've seen have been 64bit versions. Not seen any 32bit models although I'm sure they must exist. My Windows XP is 32bit. Not sure if XP does 64bit though.
I'm currently using a 32-bit system (with 2 GB of RAM) but I'd surely have a 64-bit OS and 4 GB of RAM if I'd just have money for it. I do a lot of image editing, programming etc. RAM runs out pretty quickly when you have Firefox with 10 tabs open, 5 instances of Visual Studio running and also GIMP with a large image.
I like Win 7 64 bit a lot. 64 bit is the future, after all. Pity about some flash compatibility though!