I've obtained the S550 as an upgrade over a 60GB PlayStation3 system (which was very noisy for films), and have been impressed with the offerings. I am using it with a Sony STR-DA2400ES AC receiver which provides the surround sound decoding via the HDMI port - the sound is superb, and supports both the new HD audio systems (DTS HD and HD-MA).
Picture quality is better than that of the PS3, though this can be hard to notice. Obviously the S550 supports 24p True Cinema, and has an indicator light for when this is active. Obviously this system works best on a 1080p native display, and upscales DVD to this resolution very well.
Small things I like? The remote control has a backlight for some of the main controls, the front of the player is a dark blue gloss which fits into a home cinema set up nicely. Oh, and you get two free BD discs when you register - Casino Royale and Batman Begins.
Finally, there's the ethernet jack for updating the player over the Internet (currently Profile 2.0), and accessing BD-Live content. Shame Sony don't have a wireless adapter for this though, as it adds yet more cabling to make a mess.
The long and short? Highly recommended, especially if you have a surround sound system that supports the HD audio codecs. If you don't, you can still use analogue outs and let the S550 do the decoding.