Is adapting old SLR lenses to a mirrorless camera practical and worthwhile?

Breaking down your question:

Is it worth the effort?

If you already own lenses and don't want to spend money on digital lenses, you could say it is worth it. If you don't want to fiddle with the manual focus, it's not worth it. If you have to use this in an environment where fast focusing is critical, then no, it's not worth it. This is a bit subjective.

Does focusing become difficult?

I use a Canon 550D with a Pentacon 50mm f1.8 lens. Focusing through the viewfinder is hard. With a micro 4/3 you don't have an optical viewfinder so you don't have that problem.

When using liveview, focusing is not hard at all. Just enlarge the image several times on the LCD screen and get the focus right. My Pentacon lens does have a tight focus ring, which takes more effort to turn compared to a digital lens focus ring.

Should I use manual mode?

You could use manual (M) mode or Aperture priority (Av) mode. In both modes it is important that your lens diaphragm switch is set to manual. By this you ensure that when you turn the aperture ring, the aperture blades actually move. In auto mode the aperture will remain wide open to allow a bright viewfinder image, only when the shutter is clicked it will change its aperture to the desired value. The mechanism to provide this functionality is not present on modern cameras, therefore set the switch to manual.

In the first mode you would set the aperture on your lens and set the shutter time in camera manually. In the Av mode the camera calculates the appropriate shutter speed based on the exposure it measures. You might have to adjust the exposure by under or overexposing a bit. This depends on your lens. When I use my Canon/Pentacon setup I have to underexpose with one stop to get a correctly exposed image.

Will it be practical?

When you have the time and patience to manually focus and to set your exposure settings, it will be practical. Keep in mind though that old lenses don't have Image Stabilization, so camera shake might become a problem at low shutter speeds.

When you need to focus quickly, or when you're not able to see the LCD screen when taking the shot, it's not really practical.

What is the optical advantage?

This depends on the lenses you have.