In most cases when shooting landscapes I like to be in Aperture Priority. My subject is not moveing so I have a little more time to calculate how I might want to shoot the scene. With my selecting the f/stop and letting the camera determine the shutter speed, I can move from scene to scene and maintain the desired depth of field. I almost always am shooting a landscape with a tripod so I worry much less about shutter speed.
I also like to use Manual Mode, especially if I am shooting waterfalls, rivers, and streams. In general I like water looking a bit smooth. Not too much...there are some photos that just a little too much "cotton candy" effect. I do my best to try and keep a little bit of a good thing, if you know what I mean.
I find myself in Program Mode and Shutter Priority when shooting wildlife. Depending on the subject they can tend to move a bit and be unpredictable.
I have actually had a lot of success in Program Mode. In Program I can still override the apreture with the flick of a wheel. The Canon 30D also has exposure compensation made easy by spinning an adjustment wheel.
I do some portraits and am getting into them more are time goes on. Where I shoot most people photos is at events and such. When shooting at events I am strictly in Program Mode. I need the camera ready and ready fast. Getting that "money shot" of a child takes cat like speed. I cant be fumbling around with all kinds of calculations.
I am still learning as probably most of us are. Certainly there are different reasons to be in different modes. If you have a camera that has RAW capability, switching it to RAW is not a have bad idea.
If you are in RAW you can make up for some exposure errors or miscalculations fairly easy. To some it may sound like cheating. I say Iam being smart embracing the technology.
Drop a line, give me your thoughts, tell me the modes you like and why.