I've been a fan of Cyberlink's Power DVD for some years, ever since I got it free on a new laptop I bought. Having eventually upgraded to a new machine without Power DVD, I was a bit disappointed with just using Windows Media Player.
Cyberlink's software gives you much more dedicated movie playback. It upscales nicely and has an extra upscaling option, although this is perhaps the one real negative aspect of this software as it just seems to "over-smooth" the image, a problem common among current smoothing functions in both playback software and HD TVs. I was pleased the snapshot feature is still included, as some players don't have it. This lets you take a still of any frame within the movie and you can set it to either take the original image size or the size you're playing it at - bear in mind that it will be upscaled and show compression blocking if you grab at full screen size. I have found that grabbing while the movie plays rather than pausing it to take a snapshot results in a smoother image.
The sound options are all very good, it supports up to 8 speakers and different set-up modes. I'm sure most people won't need anything near the top capability of this software, but it's there if you do.
As for Blu-ray playback, I can't help you there, as my PC doesn't support Blu-ray and I'd much rather watch them on my PS3 and HD TV, but the software does support them.
All in all, I'm very pleased with it, as I had read many negative comments about previous versions of Power DVD regarding bugs and playback issues. I didn't experience any at all, which is pleasing. The software automatically notifies you when a patch is available for free download, too.