Well, as the resident audiophile who can't actually afford any of the compenents he wants, I'll throw my opinions into the mix.
Logitech z-5500 - Pretty much the loudest multimedia speakers available. Mid-bass is overemphasized and highs aren't great. These sound better than other Logitech sets though, where the midbass sounds simply awful.
Creative Gigaworks ProGamer g500 - Haven't heard these, so I can't comment specifically, they look a bit like weaker versions of the S750 speakers. The S750 speakers are great, satellites are crisp, better than any other multimedia setups I've heard. The sub doesn't have as much punch as some other systems, but sounds great still, blends in with the sound well.
Klipsch 5.1 ProMedia Ultra - I own these. Satellites aren't quite as crisp as the S750, so I've replaced my front two with a pair of Boston Acoustics CR-67 bookshelves. Best sub of the lot, has dual 8" drivers (creative has 1 8", Logitech 1 10"). Will not sound as boomy as the Logitech sub, but matches it for volume on deep sounds, and is of better quality.
KnightRid wrote:Klipsch - heard somewhere about bad amps or something blowing out on them - might have been links through newegg.
You're correct. The original Klipsch 5.1 system was THX certified, and didn't have the "Ultra" monicker. A bunch of the THX systems had amp problems where all the speakers except for the sub would fail. Klipsch extended the warranty from 1 to 2 years on the THX model for all customers in response to this. The new(er) Klipsch Ultra speakers no longer have the amp defect.