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I bought the S1 IS from Amazon because I was looking for a camera with good zoom performance, which the newly-announced S1 seemed to have in spades.
Initial impressions were that the camera's a lot smaller than you think it would be from the pictures - it's about the size of an old-style Sony Walkman, (if you exclude the lens).
The use of AA batteries is unusual for a camera of this price - most of the competition use proprietary Lithium Ion cells. I'm using 2200mAh NiMH cells from Ansmann, which are inexpensive and charge quickly, (and so recommended), although they do discharge while stored. A fully-charged set seem to last between 350 and 500+ shots which is very useful, and at least if you get caught without power "in the field" you can use alkaline AA cells, which won't last long but are widely available. Like others, I strongly recommend that two sets of batteries are bought, which for the S1 would be eight AA cells, (should cost £25-£30).
The media used is Compact Flash, types 1 or 2, which means that Microdrives can be used, (although they're supposed to drain the batteries pretty quickly). Don't skimp on the media chosen, as 512MB "80x" speed cards can be bought for less than £50 these days. A high speed card is recommended, and the 512MB card will give over 500 shots at the normal resolution/compression settings.
The main controls are pretty obvious and easy to use, with the zoom lever being particularly good, having a "coarse" and "fine" setting both of which are very, very quiet in operation. Startup is acceptably quick, and can be speeded up by switching off startup sounds, etc. The image stabilizer, (the "IS" in the camera's title), seems to work pretty well at minimising camera shake at high-zooms, but a steady(ish) hand is still recommended.
Images are generally pretty good - well focussed; good colours - with two exceptions. The S1 lacks a 'focus assist lamp' and seems to struggle badly when the light level is low and the flash isn't used. The other issue is that the macro minimum focussing distance appears to be over 1m, which means it's tricky to get that insect or flower closeup. The vaunted Canon autofocus doesn't help matters with some point-and-shoot shots being ruined by the autofocus locking onto the wrong item in the picture, (switch to centre focus or use manual focussing).
This camera's equipped with a fold out LCD display that can be positioned to suit - great for framing that picture of yourself! There's a viewfinder available too - complete with dioptre adjustment for people lacking 20/20 vision - but it's a minature LCD screen, so doesn't offer much in the way of advantages, (saves a little power and is easier to use in strong sunlight).
A couple of reviews have commented that the S1 is heavy - I'm no "Charles Atlas", but I've had no problems shooting reasonable images with one hand. It's not pocketable, but it's not a lead brick either! The other note they usually make is that it's essential to remove the lens cap before switching the S1 on - I'd wholeheartedly agree with this, but it's common sense anyway.
I've not had much chance to try out the full range of 'professional' features like exposure-bracketing etc, but the 'motor drive' mode - where the camera takes a burst of pictures as fast as possible - seems pretty usable.
There's a 'movie' mode (with sound) which I've tried on a few occasions, and this seems not bad but it's not good enough to replace a proper video camera. Main drawback is that the movies are limited to 1GB in size, irrespective of the size of the media used. Top resolution is 640x480 pixels, which looks presentable on a TV screen, (the S1 can be connect to a TV via S-Video or RCA jacks).
The software supplied with the camera is okay, but I'd advise checking with the Canon site that you've got the latest version, as it's got a good few improvements over the version supplied with the camera, which is/was quite old.
Other items supplied with the camera are a 32MB CF card, (too small for serious use); USB connection lead; video lead; power adapter; neck strap, (loop the lens cap's lead around this to stop you losing it); quick setup guide; software disks; and a manual. The manual's pretty good and definitely worth delving through before starting with the S1.
In summary, this is a good device once it's shortcomings are taken note of, being cheap to run and producing reasonable images in most circumstances.