Hello! I downloaded ReactOS-0.3.15-REL-vbox.zip from the site and ran it in VirtualBox 4.2.16 in Windows Server 2008 R2. See my report with 4 problems I experienced. Three of them are serious. Hope this message will help you improve ReactOS.
== Problem 1 == Menu Machine->Add failed with "Failed to open "C:/Program Files/Oracle/VirtualBox/VBoxGuestAdditions.iso"". So I went to ReactOS.vbox and removed these lines: <DVDImages> <Image uuid="{38b58d16-ded5-4c11-997d-66b88780fbb1}" location="C:/Users/Z98/workspace/reactos/ReactOS-BootCD.iso"/> <Image uuid="{39d01696-2de6-4272-8bc8-0cc8836273ca}" location="C:/Program Files/Oracle/VirtualBox/VBoxGuestAdditions.iso"/> </DVDImages> (I didn't have either path on my computer.) After that I managed to add the machine.
== Problem 2 == The first boot failed with blue screen 0x1E (0xC0000005,,). Going to Settings and disabling the USB controller fixed this. I could finally boot into the system.
== Problem 3 == I did Start->Reboot in the start menu a number of times. About half the times boot failed with blue screens 0x1E (0x80000003,,) or 0x7E (0x80000003,,).
== Problem 4 == After shutting down and powering the machine on boot process failed with blue screen 0x44 a couple of times
I saved screenshots of those BSODs. Will send them if you are interested. (And I hope you are.)