The strong side of Nvidia's Ion is the ability to accelerate HD content decoding. To achieve that you need a player capable of using DXVA during playback. If you think the Windows Media Player is the choice to go, you are wrong. Microsoft is not able to do so with containers they don't like such as .MKV and even under Windows 7 WMV decoding was not as fast as it could be.
Of course we used Media Player Classics with DVXA support. It can internally accelerate decoding of H264 and VC-1. The later only when the frames are progressive, interlaced material is not common and so not yet supported. In Windows 7 we used the 64-bit flavour but our impression is, XP is the faster way to go, even when you disable all the eye-candy Windows 7 is providing.
Normal .AVI files encoded with either DivX or XviD are no problem whatsoever. All decoding is done by the CPU.
720p works even better, because Ion accelerates decoding.
With 1080p content it gets more interesting. Under Windows XP the Windows Media Player is hopeless. While the Task Manager shows about 50% usage, the movie is stuttering.
Media Player Classic on the other hand, had no problems to provide a proper HD experience.
Under Windows 7 the Windows Media Player did better, but we don't think it is, because Ion helped. We think the 64bit code did accelerate the decoding a lot. The original source is a WMV file.
Now the same file inside a MKV-container but decoding still with Windows Media Player.
The Media Player Classic x64 did not struggle, regardless of container. The first is the WMV-file:
The same file in a MKV-container:
ASRock provides an OEM version of Cyberlink so you may watch a BluRay disc. It does just work.