Its all down to personal taste, I know for me the S750's are compleatly useless since I need coax.
http://www.tomshardware.co.uk/forum/245571-10-logitech-z5500-creative-gigaworks-s750
See here its a bunch of mixed reviews, everyone has different oppinions, some people generally talking crap etc...
Some people say the S750 sub is boomy, some people say it is not, some people say they are crap for music, the Z5500 has had all these said about it, were I got my speakers at the moment on my Z5500 (its in 2.1 config for PC) I can put my ears close to the sattelites and it still seems like bass is coming from the sats even though it clearly is not since the sats cant product that, I think thats related to the fact my desk is in a corner and the sub is behind me about 1-2m away in the other corner and I cant even tell the sub is behind me most of the time.
Its all about placement, apparently the Z5500 is boomy, but all I do is turn the sub down to a normal level and its not boomy then.
The only thing making me buy PC speakers still is the price, they really are not bad for the price, I like my bass, and some bookshelves and an amp for udner 200 quid wont deliver anywere near enough compared to these PC speakers and I'm not spending loads on seperates anytime soon, I hope to get a real 5.1 some day and have a rough idea as to what I will be getting, but at the moment its staying PC speakers, as with 2.1 I cant watch movies.