I got mine on Friday night so I've had time to do a fair amount of reading on it. My first impressions:
The screen is gorgeous. I find myself still just staring at it. My K1 looked like a mass market paperback that had been on the shelf a couple years. Perfectly readable but a little dingy. I feel like I'm reading a fresh hardcover with this screen. Well, only it's a lot lighter.
It's quite noticeably faster. Granted I come from a K1 which is pretty pokey. The dictionary lookup is so fast! Page turns are even a little quicker. I have the 3G but AT&T isn't the best in my area. Having the WiFi to take advantage of my fast broadband connection rocks.
It's really thin and light. It's so comfortable to hold.
The lighted cover is very convenient. Some people don't think the light is even enough but it works quite well for me. I like that it turns off automatically when the Kindle goes into sleep mode, I don't need separate batteries, and it's right there when I want it. I usually don't carry the light with me. It stays in my night stand. So if I'm in a low light situation otherwise, I'm out of luck. Now I always have a little light.
The buttons are smaller but well placed. I thought losing the huge buttons would bug me but I like that I can hold the Kindle on the bezel and not accidentally change pages. The placement is such that it's still a mindless flick of the thumb.
Now for the negatives:
Collections are cumbersone. Maybe I haven't found it but I haven't seen a way to manage titles in bulk. Adding a book to a collection takes too many clicks. Also, I haven't seen how I rename a collection. I need to spend a little time with the manual me thinks.
Wikipedia formatting isn't as good as it was in K1. When I searched Wikipedia before, it came up nicely formatted for the Kindle. Maybe it was using the mobile version. Now it comes up in the browser looking like the full web page and I have to manually switch it to article mode. If I click on a link in a Wikipedia article, I have to switch to article mode again. It's slightly annoying.