I have an S3 IS and i like it loads , ilike every other camera it has pros and cons but for me IS + 400mm equivelent zoom + lots of manual settings + not too big or heavy made it my choice to buy for my hols.
the downsides that i found were, the poor viewfinder, like said above, its just a smaller view of the lcd, but it was easier to take shots using that sometimes when the lcd screen wasnt viewable due to bright light,etc
also it only goes to iso800, but i found iso400 to be bad and iso800 to be near unusable
i also noticed a lot of my shots looked a tad soft, easly fixable though , and i really missed having ultra wide sometimes, some buildings in hongkong and sydney were crying out for it
apart from that it served me well and i got a lot of nice photos,the live histogram is pretty cool i also found the video mode very handy
i still prefer my 20d and i`ll probably sell the S3 now i no longer need it but its a good camera especially for travelling