The sensor is cool, no doubt, but that's about it.
Sigma is like Fujifilm, they think the sensor will save them, when really it won't. The sensor might be dynamite (for 4MP), but the rest of it just isn't there.
Than again, alot of cameras look more appealing when you've got an A550, especially once you start handling them.
I don't want to sound mean but it's true. As a salesperson i see this all the time, sony just makes cheap (in the bad way) stuff. People come in and it goes something like this:
"hey can i check out the Sony A330?"
hands them said camera
"can I also see the D3000?"
hands them said camera
"Wow, the Nikon feels alot sturdier, why is that?"
ALL of the sony's, even the A900, feel like hollow plastic fisher-price toys compared to Nikon, Canon, Olympus, and Pentax. They even sound like their going to fall apart.