I got the same Sony sound bar for Christmas last year, and it's been a major improvement over the awful speakers on my Bravia. It has ample sound for pretty much all uses, but don't expect much more. For $250 it's a convenient solution to the tinny speakers on most new TV's though.
As for connecting speakers with HDMI, I'm not really sure what you mean.. Are you asking if it's better than using like digital optical to connect from source to receiver? In that case the answer is definitively YES!
A sound bar is going to give you simulated surround, and most of them sound pretty good. A center channel speaker is going to only going to act as a standard speaker, and your going to need something in between the speaker and tv to be able to produce the sound, a soundbar will (should) be able to connect directly to most TVs.