I've had the X100S for a couple of days now and I am enjoying it immensely. Here are some of my initial impressions and thoughts. Bear in mind that I don't have any experience with the X100. I use the Nikon D90 as my main camera and was using the Ricoh Digital GRIII as my carry about.
When I took it out of the box, I was surprised at the heft, even though I had held both the X100 and the X100S in shops. I guess hearing so much about the lightness, compared to DSLRs altered my expections plus my previous carry about was a compact camera. I've gotten used to the weight and it feels almost just right, maybe a little heavy.
Next is that it is already doing its job. I bought it to document my life, taking snaps of my family and friends will probably take up 2/3 of its frames. Sounds crazy to use such a camera for snaps, but its something very important to me. I have already taken shots that I wouldn't have been able to with my D90 or the GRIII. Most people including my kids largely ignore the X100S. If I whip out the D90 even with a 24 f2.8 there's a different dynamic. Ergonomically the GRIII was great but its small sensor let it down. I always felt I was fighting the camera to squeeze out the best image quality. Especially indoors at high ISO, most times I came out on the losing side. So much so I stopped carrying it with me.
The X100S is really impressive in this regard. ISO 2000 looks great, and I do find I need it a bit. It's better than the D90. And it isn't a noise factor, it is dynamic range. Past 1000-1250 the D90 files looked sickly and unnatural even when properly exposed.
Other thoughts, the manual focus ring still has the fly-by-wire limitations. It is nowhere as responsive as a direct coupled lens. The split image aid works but depending on what it's over, it can be equally hard to judge if the two halves line up.
The AF-C mode on the other hand has impressed me so far. For keeping the focus on a moving child's eye, it works as well as a DSLR.
I heard stories about the slowness of the SD card handling but so far I can't see any issues. I was about to buy a San Disk Extreme Pro but I'm not sure I need it. I am using an EyeFi card that is only rated Class 6.
Oh, one piece of bad news, I have managed to freeze the camera once. I had to take the battery out to solve the problem. Switching off via the switch didn't have any effect. I think it was because the AFS/AFC/MF switch was in a in between position when I took a photo.
Feel free to ask my any questions. And don't worry, I am intending on using it as a creative and professional tool as well. Delighted to hear about LR4.4