Apart from it being a measurably slower architecture (15-20% is typical) and the only sane way of dealing with large memory machines? (Laptops with 8G are routine today, and require a PAE kernel on i686). Refusing to upgrade is the reason that products like Skype continue to refuse to support 64 bit targets.
Just try it. Chances are you won't even notice the difference, except that your battery will last a tiny bit longer if you do compute-heavy stuff.
Tried it couple years ago and the only thing I noticed was that it used more memory (almost twice as much as 32bit IIRC). So I returned to 32bit and am still using it, with PAE kernel so I can use all 4 gigs that I have right now. I really don't feel a bit responsible because Skype refuses to support 64bit.
Another reason that I keep 32bit is portability, as I have quite a peculiar setup: I keep my Linux on a removable HDD, and boot off it on several different machines, which may or may not support 64bit. (So I don't have to tow my laptop to work and back every day.)
You can run a 64 bit kernel, with a 32 bit userspace BTW, this lets you access all the memory on your machine.
Personally I'm waiting for debian multiarch to switch, since I have no desire to reinstall everything from scratch. If debian multiarch did not exist I might have bitten the bullet, but since it does, I can wait a bit for it.
But those are still bleeding edge options for people comfortable with tinkering with their systems. I was speaking to the simple choice of i686 vs. x86_64 in pre-cooked systems.
Just try it. Chances are you won't even notice the difference, except that your battery will last a tiny bit longer if you do compute-heavy stuff.