The Logitech X530 is supposedly nothing special. Like the Z623 was inferior to the Z2300 it replaced, it's inferior to the Z5500 it replaced, which itself was rather overpriced for what it offered (though like the Z2300, it was better than much of today's Logitech stuff and was relatively "good", but both are said to have their flaws). 5.1 sets that cost less than 2.1 sets do for a reason. You get less. You're far better off with a good 2.1 setup.
Anyway, those reviews are probably from people who know little better. Honestly, the Logitech's aren't going to be "bad", and if you're like most people, you'll probably even love them. Between the two, I'd recommend them. I would think they're not much worse, if at all, and they likely have better support. Still, there's far better options out there though. I have a set of Z623s and they're "alright", but I wouldn't buy them again personally (I actually didn't but them; I bought Z2300s and when they failed, I got these, but honestly, I probably wouldn't buy the Z2300s again either). If you've never heard better and aren't an audiophile though, they'll probably work for you. Audio is really relative until you start reading up (and listening up) on it.