Regarding SMP stability:
Instability with use of 2 virtual processors or more. It will usually load up and run for about maybe an hour or two. After that however, I either get Kernel Oops, kernel hangs or a hard lockup. Compiling even with conservative cflags usually results in a hard lockup or kernel oops requiring me to reboot to complete compiling.
When running with only 1 processor, the beta seems very stable. Thus far, I haven't had any crashes or lockups and no errors when compiling when using 1 processor
Host :
Vbox 3.0.6 beta 1 release
Windows Vista x64 SP2
Intel Q6600 @2.4
NVIDIA 8800 GT
Guest:
Gentoo Linux x64 with KDE 4.3.1
Guest Additions installed