Had a "less than seamless" upgrade to Windows 10 on my Alienware X51 machine (least important/first upgraded). After a couple of hours trying to resolve the issue myself (it would boot to a black screen with a mouse pointer), including wiping the machine and starting over from scratch, I called Alienware support. I was pleasantly surprised at how competent the support guy was, although it took another two hours of the two of us working together to resolve it.
Ultimately, the issue seems to be a bug in Windows 10 that was causing it to think that the onboard graphics card was connected to a monitor when it wasn't, and that the discrete graphics card was a secondary monitor. After I figured out that I could hit shift and type in my password blind to logon, we were able to force it to think that the discrete graphics card was the primary monitor and it seems to be working.