I have a Canon with Evaluative, Spot, Partial and center-weighted.
First, it's important to understand what each of these does. I'm describing them from memory, so if I get one wrong, I hope someone corrects me.
Evaluative takes values from the entire scene and attempts to pick an exposure level that gets the largest amount of the picture properly exposed.
Spot metering meters off of a very small spot in the center of the frame.
Partial metering is like spot, but it uses a larger area to meter off of.
Center-weighted meters off of the entire frame, but it puts more emphasis on getting things in the center metered correctly.
Another thing that is important to understand is how the camera's meter works. Metering has nothing to do with color. The meter works as though you are taking a black and white photo. The meter assumes that everything is grey. When it sees something white (or very light colored), it assumes that it is grey that is very brighly lit. When it sees something that is black, it assumes that it is grey that is very poorly lit.
In most situations, I would recommend just using evaluative. It works reasonably well for most stuff. When it is off, you should catch that by looking at your histogram (you do check that pretty frequently, don't you?).
If you find yourself taking photos in somewhat tricky lighting and still don't want to mess around a lot with exposure, switch to center-weighted. It will handle situations like back-lit subjects better than evaluative. The disadvantage is that, when taking a lot of shots of the same general scene, you exosure level will tend to change more. That will happen because the exposure is based more on a smaller part of the picture and that area is likely to change more often. When looking at a lot of pictures of the same scene, it is often better to have consistent exposure (and white balance) than to have each picture adjusted individually to fit the subject.
Use Spot or Partial when you either have enough time and desire to get the exposure just the way you want it or you're taking a picture of something that isn't working with evaluative or center-weighted. In it's simplest form, spot metering allows you to pick the one thing in the picture that you want "correctly" exposed. I used the scare quotes because the meter will expose it as though it is grey (a shade of grey called 18% grey to be exact). If you put the spot on something that is a dark color, the meter will tell the camera to overexpose that spot so that it looks like 18% grey. If you put the spot on something that is light color, the meter will tell the camera to underexpose that spot so that it looks like 18% grey.
If you have a lot of time to set up a shot, look at all of the major elements in your picture and use the spot meter off of them. Assume that your camera can handle about 6 stops from the brightest to the darkest part of the image (the actual range varies from camera to camera and decreases with higher ISO). Use your spot meter readings to decide what exposure will keep everything you want within that range. This technique lets you choose what will be exposed and what will be blown out or lost in the shadows. To be honest, unless you are shooting something with action in it, it's easier to just fire off a few shots at different exposure levels then pick then one you like best later.
The lazy (and often efficient) man's way to proper exposure is to just look at the darn histogram and take the picture again if you need to. If the histogram isn't bunched up on either end, you're probably good to go. If you are hitting either end, adjust the exposure so that you aren't. If you are hitting both ends, you've got to make the decision as to whether you want to lose more detail in the highlights or the shadows.
If you want to picky, you should try to get the histogram as far right (brightest) as possible without hitting the right edge (which means you blew something out). This is because sensors work best when they are getting the most light that they can handle without going over (think The Price is Right). This often leads to too bright of a picture, but you can correct that easily in post production and you'll have less noise than if you'd shot it correctly to begin with.
One big warning about histograms is that you need a color historgram and not simply a B&W histogram. A color histogram will show you a seperate curve for each of the colors. It is possible to have the average light level look OK only to find out later that you've blown one of your color channels. This happens most often when shooting saturated colors. If you don't have a color histogram, you can still use it, but leave yourself more margin for error.
Well, that was a long-winded answer to a short question.
