Thought that I'd weigh in my opinion since I recently picked up a set of Z-5500s. In fact, it was partly due to threads like this one that made me want to try them out. Its too bad that I don't still have the Gigaworks with the DDTS-100 to compare it to, but I do have my home theater system comprised of the following: Onkyo TR-602 Reciever, Dayton 10" sub, Polk CS1 center, Polk R50 fronts, and Polk R15 surrounds.
Now I really like my home theater system. Its not killer, but I think its quite good, especially for the price (I paid about $700 shipped for it all). For watching movies its absolutely stellar, with one exception. I have to turn it up before it really starts to sing. Now, this wouldn't normally be an issue, as especially for some movies, the louder the better. However, I unfortunately live in an apartment building, and so I refrain from watching movies mostly because I don't want to bother the other people since I have to turn it up to really get much out of it. Music is good, but also suffers from needing to be turned up. I had an SACD capable DVD player for a while and at low volumes it sounded just kinda ho-hum, but once I turned them way up it was very good. Games are ok, but not really exceptional.
So I hooked up the Z-5500s to see how they'd sound in the front room. I figured I'd need to turn the volume up about halfway to get them to make much sound, but I was surprised that I had to turn them down, to about 2 bars (5 or 6 being about halfway), as they were quite loud. They were nice and clear even at the low volume level. I actually even had to go adjust the sub as it was a little too punchy compared to the speakers at the default level. I was liking them. Listened to some music (several CDs I had picked up recently), and was again impressed. Then I figured I'd check out a couple of movies to see how it'd compare there. This is where I finally saw a weakness in them. War of the Worlds has a scene relatively early on in the film where the bass is just astounding. In fact once I heard it on the Datyon, I was fully convinced at that subs greatness. You could really feel it, and even if I turned it down, the thing would still reproduce a great sound/feeling. I figured it was probably one of those very low frequency sounds that people were so impressed with the Dayton's capability to reproduce. So I go to that scene, and find that (not surprisingly) the Logitech sub can't match it. I turn the sub up higher than it was default and it still can't muster the same feeling, although it certainly was putting out a very decent sound.
Now, I was intending to use the Z-5500s in my room (which is considerably smaller than the front room) and so I'm sure the bass will be fine for that. However, I have to say that I am actually planning on replacing my home theater setup with the Z-5500s. The reason is that the clearity for the volume difference makes a world of difference for me in the apartment. I'm actually glad that the sub can't match the Dayton, as I'm sure my neighbors will be as well. That and I'll save myself $500 or more (should be able to sell my HT setup for maybe $800-900).
Adding more to the having to turn my home theater setup high, I actually had taken to listening with my Sennheiser HD-555s to hear things. The Logitech have a kinda comparable sound in my opinion (although I will admit the headphones sound better). Its a little overly bright, but I have to say I like it.
So, like has been pointed out sound is very subjective, and also, in some situations, its just not very feasible to go with a nice big full speaker setup.