Can I use Graphical User Interface (GUI) all the time?

You can. But I don't think it would be very efficient for administration. The GUI under Linux is very nice, but it pales in comparison with the possibilities available under the command line. I do use the GUI every day both under Linux and MS Windows, and I do think that GUI great for program launching, navigating standard documents, and other routine tasks. Yet, for system housekeeping or automation, GUI is inflexible. The command line interface (CLI) is a richer interface, and it also gives me some understanding of the working of my system.

Two quotes to amplify this point: "Using a graphical interface is fairly easy because it limits you to a few basic operations that you can learn quickly. But if those basic operations aren't what you need, then you may have problems". "One characteristic of a user-friendly system is that it does what the user wants. In other words, the most user-friendly system isn't necessarily the simplest one." [http://linux.oreillynet.com/pub/a/linux/2001/11/15/learnunixos.html]

I find that a mix of GUI and command line is perfect to cover all my needs. If you are determined to use GUI only and are not willing to learn any command line options, you might consider delaying your Linux installation until the GUI tools are better developed, unless you have somebody to help you administer your computer for now.

On the other hand, if you have a computer-agnostic girlfriend or boyfriend, and all s/he does is execute perhaps half-a-dozen different programs, you can set up a nice GUI screen for him with the icons or buttons or menus he requires. This, after some intial encouragement, may make like Linux.

From this Guide's point of view, command line is certainly simpler for documentation. Icons and menus are meant to be customizable and therefore your icons and menus may be quite different from mine. Also, to set up an icon or command, I need to know the command that stands behind the icon. In short, understanding of the command line is indispensible for setup and any work beyond trivialities, even under GUI. We include no screenshots in this guide--we found them ourselves useless in learning about computers.