I like it showing.
If I have it hidden, it pops up as soon as I put my cursor near the bottom of the screen, which irritates me. I know it is meant to this, but when I do it by accident it irritates.
I prefer to have my windows resized so they are aware of the dock (or dock-aware as I call it). Plus if you want to show it for a moment, then parts of the window get hidden - (normally the scroll bars).
If the dock is hidden and you want to drag documents to launch them in a particular application (say a psd in safari or preview), then you have to drag the icon right to the bottom of the screen, wait for the dock to slide up, then move it over the application you wish to launch it in. If the dock is showing then you can just drag and drop the icon right over the app.
I can't get used to it on the left or right. I hate it when the icons are small (This has a lot to do with the fact I don't like it on the left or right). Magnification is fun, but not really useful for me as I have the icons at a nice size anyway (my dock is about 60px high).
I also like seeing the status. i.e. number of mail messages, whether transmit has finished an upload, whether photoshop has finished a filter. You miss out on that with a hidden dock and tiny icons.
If I am doing quite a lot of work in one particular app (like Photoshop), then I will sometimes hide it. But it is unusual.
Another thing - applications *have* to bounce when loading. I tried turning it off, and all you get is a flashing triangle which is barely noticeable.
One more thing - genie is my preferred effect. Scale just isn't the same.