I owned five Powershot S Series (s30,s50,s60,s80,s90) cameras prior to purchasing the s100. I took close to 100,000 photos with my s90 in the two years that I owned it. I am a semi-professional photographer that owns multiple Canon EOS DSLR cameras with L lenses.
First off, I don't think it's fair to compare the s100 to DSLRs, APS-C, Four Thirds cameras. These large sensor cameras are in another class and even my really old Canon 10D with 6MPs will at low ISO have better image quality than the s100. If you don't mind a larger camera that is not pocketable and are willing to spend more, the s100 is not the best camera for you. For example, the Sony NEX-5N will give superior results to the s100 in nearly all situations.
The s90, s95 and now s100 are all unique in that the are truly pocketable--all the competitors are too large for pant pockets (Panasonic LX5, Olympus XZ-1, FujiFilm X10). Some of these cameras are quite good and all of them provide faster lenses across the zoom range (the s100's size precludes a faster zoom lens). But none of these competitors are comfortably pocketable--this means that the s100 has practically no competition for it's size as a truely pocketable semi-pro camera.
If you don't need the manual control, the Canon 300HS will also take great pictures for less than half the price in a smaller/slimmer camera body. The s100 does have a great AUTO mode and multiple Scene modes which in most situations does an excellent job. But shooting both the s100 and 300HS in AUTO will lead to very similar results. The s100 differentiates itself by providing excellent manual control and the ability to shoot RAW images.
Now if you are someone that wants pant pocketable camera with excellent manual controls and image quality, the s100 is very hard to beat. I take ten times more shots with my s100 than my DSLRs because I ALWAYS have it with me.
If you already have a s90 or s95, there is little need to upgrade to the s100 immediately. The s100 has some worthy upgrades (24mm, GPS, better designed body) but the image quality is near identical to the s95/s90. My s90 took many falls and goo in an airplane pocket killed the flash otherwise i'd stay with the s90. I have now taken close to 1500 photos with the s100 and here are my experiences of it's pros and cons:
PROS: * The s100 is cleaner at high ISO (eg 1600 and above) but the difference isn't as substantial as Canon claims (for low ISO, there is no difference in noise) * s100 is even slimmer than the s95 which is was already quite pocketable * GPS is a great addition and quite accurate most of the time * rough texture finish makes the camera easier to hold and also looks great * rubber grips make camera easier to hold but don't expect miracles * mirror like shutter button is easier to find especially given the removal of the func button from the top of the camera * back wheel now has the perfect resistance and feedback * the delete button is not on the dial so I can finally delete while viewing a zoomed in on an image!!! * lens zooms very quickly (sometimes too quickly to get accurate zoom) * fast shutter speed (in fact in Continuous drive mode it is very difficult to take one photo--always get two photos and I am fairly fast releasing the shutter button--an additional slower Continuous mode option would have been helpful) * 8 shots in a second is a handy feature to have though not often used (limited to 8 shots and then have to wait for the photos to be written to the card) * 24mm wide-angle is very useful for indoor photography * lens is sharper across zoom range than the s90--with sharper corners (people have reported variation in the sharpness of the lens on early models) * ability to control max ISO and ISO selection criteria for Auto ISO * dynamic range correction works well * dedicated movie recording button * 1080p video recording (much better resolution but not as good low light capture than s90&s95 but can't match Sony HX-9V) * can zoom while recording video with minimal noise
CONS: * all Canons Point&Shoots have slower focus especially in the dark. The s100 is comparable or slightly slower than the s90. Fuji and Sony have made strides to provide faster focus and I hope Canon follows. Set shortcut button to Focus-lock to pre-focus--once focus is locked the s100 is really fast * NIGHT PHOTOGRAPHERS: a major con for me is that the s100 is locked to ISO80 for shutter speeds over 1sec. May not matter to most people but I loved shooting 15sec ISO400 shots with my s90 * The display does not brighten as much in a dark room making composition more difficult. The s90 shows a very grainy but bright image in a near dark room where the s100's display is very dark (also if multiple shots are taken with the shutter button half pressed the display is darker for the subsequent shots) * The s100 is better in high-ISO short shutter speed situations but the s90 is better for long shutter speed photography * Auto mode is limited to 1/8 (have to switch to P mode if you don't want to use a flash in a dim room) * HDR requires a tripod because camera does not align images (given the tech Canon really should auto-align the images). * battery life slightly worse than s90 (buy a second battery--I had three for my s90) * fast zoom with steps sometimes makes it difficult to get the exact zoom desired * Movies are great in well-lit areas but have a lot less gain than the s90 resulting in hi-res but darker movies * Sony's panoramics are fantastic and Canon is still using same approach as it did in the s30 in 2001! * always open flash if it is needed before handing the camera to anyone else. Everyone initially presses down on the flash resulting in a flash error. * price but you get what you pay for
UPDATE 03/25/2012: Had to call Canon Support for the first time in a decade (lens error on s100). Discovered, that you have to pay to mail back the camera to their facility in Virginia and be without a camera and wait 2 weeks to get it back. Maybe I have been spoiled by the excellent warranty Amazon provides for it's products.
UPDATE 05/31/2012: My Canon Service Center experience was much better than I expected. They kept me informed on each step of the process and got the camera back to me within a week. Yes, I had to pay to mail the camera to Canon and be without the camera for a week but the overall experience was excellent. (Have taken over 12,000 photos with S100 and still love it).
SONY RX100 Announcement 06/05/2012: Sony has just announced what I consider to be the first pocketable camera that delivers better features and image quality than the Canon S95/S100. The only major con against the RX100 is that it costs $650. For that price you get a larger sensor, better iso performance, better video, much better auto focus, better features, ect. If price is not a concern the RX100 which will be released in July is the best pocketable camera to get based on early impressions of reviews and image quality comparisons. Sony DSC-RX100 20.2 MP Exmor CMOS Sensor Digital Camera with 3.6x Zoom
UPDATE 09/18/2012: The Canon S110 was announced yesterday and is mostly unchanged from the S100 with the following notable changes: Wi-fi functionality, touchscreen, GPS functionality removed, presumably faster focus. Will cost $450.
I will soon update this review to update it based on the options available today since there have been a number of changes in this camera segment in the last year.