My experience with cameras in this class started with a Canon S-100, which I upgraded later to a Canon S-400, both of which proved to be excellent for my on-the-go photography needs. My only criticism with this class of cameras was that they lacked two features I really wanted:
1. wide angle lens
2. image stabilization
I guess the Canon folks read my mind with the introduction of the SD-800, and after reading a number of positive technical reviews for the camera on the web, I purchased one as an upgrade for my beloved S-400. I was particularly suspicious regarding image stabilization, since my only prior experience with a camera using this technology had been disappointing (Canon S-1, a larger 10x zoom model).

As might be expected with the advances in memory technology, the SD-800 takes movies at a higher resolutioni (640x480) than the S-400 (320x240) and has a higher maximum pixel count (7.1 vs 4.0). It also uses the smaller SD memory cards as opposed to the bulkier CF cards for the S-400, and the lithium battery pack has a different form factor (which means you can't reuse stuff from an earlier model).

What I consider to be the 2 real upgrade features for this camera, the wide angle lens and the image stabilization, are what truly distinguishes it from its competition. I've had this camera for about a month now and can happily report it is a significant advance over the S-400. The image stabilization is a dream. With the S-400 I had to rely on bracing the camera in many circumstances where with the SD-800 I can count on sharp images just holding it out, composing, and taking the shot. In a recent work session where my group had accumulated a great deal of writing on a wide white board along the side of a narrow conference room, I was able to capture in a single, sharply focused shot the entire board, something the S-400 would have taken 2 shots to achieve followed by a photo stitch. There is some distortion at the outer edges, but I personally don't see this as a drawback given the advantages to getting the whole image (after all you can crop the picture if the rather small amount of distortion truly bothers you).

What else? Well, the camera is lighter and more comfortably contoured than the S-400. It does retain the view finder (thank heavens ... there are simply circumstances where this is the only reasonable way to compose a shot). A single door is used for both memory and battery (vs. 2 doors on the S-400). As others have reported, the door has a flimsy feel, but my first camera in this line (the S-100) had a similar door and I never broke it. Anyway, the USB-2 picture download pretty much eliminates needing to pop out memory cards to get a faster download speed via a card reader. I did invest in a 4Gb high speed SD card since maximum movie length is 4Gb at 640x480, 30 frames per second, or about 25 minutes worth of pretty darn good movie taking. My experience to date is that with this level of capability and convenience I doubt I will every use my cam corder again for family movies.

Out of the box I found the controls and interface to be easier and more intuitive than the S-400. I have all the capabilities I had with the S-400, plus some "gee-whiz" features I've had fun with but I doubt I will use routinely (e.g., color swap). I can also recommend Canon's leather case for this camera. It provides an extremely easy way to carry the camera on your belt (it uses a belt loop, which I think is far superior to a belt clip). I was initially put off by the magnetic flap used with this case, but now agree it's a superior design (their earller cases used Velcro).