I like the theory of UEFI and it's about time that GPT standard gets more widespread. It was available with windows Vista and server 2008 but the way it was done in those OS does not seem compatible with Windows 8. I tried setting up a drive several times with Vista 64, server 2008 64, Windows 7 64, Server 2008 R2, Windows 8 64 and Server 2012. In theory it should have worked. In practice it did not. I could get vista and Windows 7 to play nice together, I can can windows 7 and windows 8 playing nice together. But all 3 was a no go.
Switching booting between legacy and UEFI mode is painful. I'm not sure if it's worse on a Dell because I don't care for Dells, and I've been arguing with dells for several days now. I had setup one Dell T1650 with one drive in UEFI mode for Windows 7 64, Server 2008 R2, Windows 8 64 and server 2012, the other drive had all legacy OS, all 32 bit OS and of course that vista 64 and server 2008 64 that were not compatible in UEFI mode. This second drive was in legacy mode and of course MBR. I use XP to partition the drive. I had switched back and forth between both drives without any issues, and the system went - when it came back it was no longer booting on the UEFI drive. Kept saying no boot device found.
Then there's the Dell T1600 where I had a drive with all OS between XP and server 2008 R2, in leagcy mode, and I setup Windows 8 and server 2012 in UEFI mode. It was working fabulously until I lent it to a less tech savvy colleague. Now it will boot into it's drive if I'm using the onboard graphics card, but won't boot using an add-in card to a hard disk. It will hang. Since I need to test it with add-in cards it won't be particularly useful to the intern who's supposed to have it as a test machine.
It;s weird too when you switch between UEFI and legacy mode on the T1600, it tends to lose all boot devices and goes to this weird default legacy mode, and if you want to set it in UEFI you cannot go between you have no options. It's just really weird. It's like no one thought it through.
HP seems to handle the switching between both better. My Lenovo can be incredibly bitchy as devices appear and disappears as well in 'UEFI" mode.
Another interesting discovery one of my interns made with build 8441 of Windows 8 is that if you were configured in UEFI mode and ran Worldbench 6, it would trash your hard disk. I've since confirmed it with Windows 8 RTM. It trashed my hard disk. I wonder what other software will also trash my hard disk in such a way that I can't reghosted it using the 'partition' method. So far that method has worked really well for me. You can run worldbench 6 in non UEFI mode and it won't trash your drive but it may not run particularly well.
Something tells me that my near future will be filled with me restoring drives that get trashed in GPT mode. So far it's happened several times on a fairly small sampling.