I would like to use the 64 bit version of OpenBSD, but I'm not really sure how good it works.
Are there any known problems? Is the 32 bit version more recommended for productive environment?
According to the official platform description:
Another mentioned limitation is that if your processor lacks the NX bit (most amd64 processors have it), on a 64-bit system, you won't get OpenBSD's protection against some exploits based on uploading executable code as data and exploiting a bug (e.g. a buffer overflow) to execute that code.
There are several virtualization technologies that allow running OpenBSD/amd64 and OpenBSD/i386 on the same machine (Xen, Vmware, and VirtualBox should support OpenBSD/amd64 guests with a 64-bit hypervisor or host OS; I don't know if there's a way to virtualize OpenBSD/i386 on an OpenBSD/amd64 host).
I have used 64bit OpenBSD as a firewall and router without problems for over 2 years, and would highly recomend its stability.