I have never been to Tweeter, but just looked at them online. They seem to be a little more upscale than Circuit CIty/Best Buy/Future Shop. They carry a few lines and models that cross out of the mass market stuff.
I totally agree with the more specialty shop comments from above. The salespeople typically are genuine music fans, or should I say those who want to spread the hobby. They usually take the time to setup decent systems, will spend the time to answer questions and demo different pieces, and are much more knowledgeable than the people in the big box shops.
If you ever want a treat, visit a Linn dealer. Their principles for demonstrating speakers is to have only ONE pair in a room while a system is playing. This can be interesting, as when I wanted to here the Kanns, with the Isobariks (utter monster speakers) still in the room. I said to just move the big guys aside, and he said no, and dragged them out!!!
