I dont think discussing all the features would be a good idea here, as there are sites like DPReview (https://dpreview.com/reviews/specs/Canon/canon_s5is.asp) where you can find review by professionals.
What I want share is my experience as an amateur photographer.
I have had an Canon A 520 for two years. It is a great camera, but I wanted something more as there are a few things that I couldnt do with it. For example, getting real close to flowers and real close to the moon.
I had Canon S3 and S5 in my mind. I did some research and found that the price difference is not much but s5 has a faster processor, face-detection-focus, flash hot shoe and a few other things. I decided to go for it.
I bought this camera. Just see what kind of pictures I am shooting at https://picasaweb.google.com/eupendra/TheS5ISExperience and decide yourself.
And about Panasonic FZ18, yes it talks about 18x and wide angle. But when you talk about picture quality, no one comes even close to Canon S5.
I could have gone for a SLR. But then, thinking about an SLR with a 400mm lens, it will be damn costly and heavy to carry around.
EDIT
A few things I like about s5
Excellent movie quality. I have not touched my handcam after buying this. It has a dedicated movie button so I dont have to change modes to shoot a movie. The zoom WORKS in the movie mode. And most important feature... I can take still pictures while I am shooting a movie, without stopping the movie.
super macro mode. I took pictures of flowers and straberries when they were literally touching the lens. With 8MP, you can crop a little and get a effect that SLRs would a need a macro lens to get something similar.
slow sync and second curtain flash modes - The night shots are wonderful. The background comes crystal clear and the faces come crystal clear. Though the camera shakes come into picture and you better place the camera on a wall or still better, buy a tripod.
-lens mounting threads. you can accessorize the s5 as per your needs. I have bought a polarizer, a UV filter, a 1/2 ND filter and a sun hood. I am planning to buy a 2.2x tele photo lens. I will then have an amazing 26x zoom.
-flip LCD. Gives me a lot of flexibility. I can place the camera on the ground, flip the LCD so that I can see easily what I am shooting. I can turn the LCD to my side and take self-portraits.
-color accent mode - Allows to leave only one color in the picture and make everything else greyscale. Gives amazing effects.
- uses AA batteries, not alkaline. I never run out of batteries as I always have 3 sets of batteries with me.
-support for SDHC card. I have a 8GB card in my camera.
Few things I dont like
- Higher ISO (anythign higher than 200) is useless. Leaves grains.