Here are my DSi first impressions after a few hours with it:
The screens are big, but otherwise about the same quality as the DS Lite. The bottom screen still has a slight tinge to it and the very slight "greasy" look just as the later model DS Lites. It appears that everything I read about the DSi screen "fixing" this problem is either due to uber elite review models or reviewers who don't know what they are looking for. I wouldn't even know myself if I hadn't personally owned an early model DS Lite which had a crystal clear, pure white touch screen.
The screens' refresh rate seems the same as the DS Lite. Despite the larger size, there is no excessive motion blur. Overall, just picture the DS Lite screen, but slightly bigger.
The sound is clearer, but maybe just because it is a little louder. However, the speakers (or built-in amplifier) appear to be prone to distortion. If I turn the volume down a bit, it minimizes this, but at full or near-full volume, even the simple DSi menu sounds are distorted. I haven't tried headphone output yet, which I guess would immediately reveal if it is the amp or speakers at fault.
The d-pad and buttons are "clicky", like the GBASP. I never had a problem with the Lite controls, other than comfort, so I can't tell you if the DSi is "better".
It is weird to use the apps like Download Play and Pictochat and suddenly be thrown back 5 years to the old DS interface look with the large pixelated clock at the top and different text font. It is literally the exact same app taken directly from the DS Lite, but with the some text strings changed to reflect the "DSi menu". I guess they were too lazy to update these.
The wifi settings maintain the standard connections 1, 2, 3 which only support WEP. I assume this is for backwards compatibility. If you try to use the wifi autodetect feature on a WPA access point, you will get told to use the "advanced settings", which are back on the main internet menu. You then have to dig through that menu to get to connection 4, 5, 6 which are the ones which support everything up to WPA2 AES.
Web browser looks nice, but as mentioned, no flash and it isn't very speedy. It is much faster than the old DS browser, but still has very slow rendering. Despite apparently keeping the whole web page is memory, it only renders the exact portion of the screen you are looking at. So when you scroll, you are stuck looking at a bunch of white space as the DSi swaps in the new portion of the web page.
The cameras are fine. The images look just as good as my MacBook webcam. Obviously you aren't going to be going around taking wedding pictures with this thing, but for its intended purpose (quick action shots or portraits) it works fine.
The biggest surprise about the DSi is that it comes out of the box with a full battery charge.
The DSi is also surprisingly larger than the Lite, despite losing the GBA slot.
To sum up my experience, other than the DSi Ware, there is almost no point in this "upgrade". I was keeping the faith that the larger screens, superior speakers, and new menu system would be worth $100 (difference between selling my Lite and the price of the DSi). I am sorry to admit that it is not. The screens are larger, but are virtually identical in quality to the Lite's (I did several tests side-by-side). The speakers appear to be larger and the new volume control is nice, but the amp/speakers being overdriven ruins it. The new menu system is nifty, but it is laid out in a 100% horizontal fashion which means you will be scrolling many pages left and right to find what you want after only downloading a handful of apps.
Until some must-have DSi Ware or GB/GBA games are available for download, I see no reason to spend any further time or money on this device. Hence, I plan on keeping mine for a few more days as a test run, then likely returning it for a refund.