Just to add my 2c...
I have been using Aperture Priority most of the time, and I am slowly getting into manual focus. I was finding it very difficult to get good manual focus, but if you have a fairly fast lens you might want to have a look at this:
Haoda Screen
I have recently bought one of these beasts for my Canon 50D and I now find that I can perform manual focus with good accuracy. There have been a number of discussions about the screen affecting AE on the camera but I have not been able to discern any difference.
The only downside I have found is that for a lens f/5.6 or slower the split wing is dark in one half, however because auto focus continues to work perfectly I am not loosing anything when working with these lenses.
It's early days for me yet in its use, but I am very pleased with the screen.
Regards,
Steve