After using Vista x64 RC2 for a few weeks solid now, I've come to know it rather well. Overall, I'm fairly impressed. For example, I'm enjoying the built-in packet writing support for optical media. What that means is that you can quickly "format" a CD or DVD (R and RW both) with a compatible filesystem (UDF in this case) and write to it like you would any other removable media. This is something XP needed before it launched. Gone are the days where you need Nero just to write some files to a disc. I also like the new interface. It's smooth and polished. It really just feels like you're flowing with the OS more. On that note, a lot of common options and tools have been changed, but they are still easily found. Overall, good stuff.
There are some things, however, that I don't like.
1) Memory usage. At any given time, Vista is using about 1.5GB of system memory just for itself. While some of this is just cache that will be sacrificed when a userspace app needs more RAM (like a game), much of it is not. This seems kind of excessive.
2) DirectX 9.0 backwards compatibility. This still needs some work. When combined with insufficient RAM outlined by #1, objects start disappearing in games. I saw this happen in both Battlefield 2142 and Counter-Strike: Source. Some of the larger maps in CS triggered this. Frequently, this happened in Battlefield. I am guessing it is a timeout issue that is exceeded by paging operations to get the desired textures, etc, when out of physical memory. DirectX windows used by video playing apps based on DX9 force Vista to use "Aero Basic" while the program is running. Essentially, the 3D interface for Vista is disabled and the 2D one used during this time. It's annoying and from my understanding not necessary. Perhaps this will be addressed before launch.
3) Account priveleges. While good by its intentions, the current status of this feature is rather sad. If it just asked one time before granting administrative access to your machine for a program, that would be OK. However, a typical installer will throw about three warnings that you have to provide feedback on before the installer will even launch. This is quite annoying.
4) Signed driver requirement. I do see the benefit of requiring signed drivers, but until driver support improves this is a real hassle. Many people have to use XP64 drivers for some of their hardware, me included. This always makes Vista throw a fit, and you must use the "Disable signed driver requirement" boot option every time you load Windows. There really should be a way to permanently disable this requirement (at least prior to Vista's launch).
5) Poor 3D driver support. This is ATI's and nVidia's fault, not Microsoft's. OpenGL support for ATI hardware is nowhere to be found at the moment. nVidia recently released a new driver for RC2 that adds OpenGL, but ATI is still stuck on their RC1 driver. No Doom for you, apparently.
Overall, I'd rate Windows Vista Release Candidate 2 at 6.5/10. It's showing good promise, but there's still a way to go yet. I'm a little shocked by the current state of Vista, considering this is a second release candidate. Many of the small bugs (incomplete dialogs, etc) should have been taken care of in the beta stage.