I actually used the above site a few months ago when I wanted to learn how to do this. It was quite fun. I spent hours importing users with all sort of AD fields populated. It made me realize how sophisticated it makes your directory when most of the objects' attributes are populated.
A few other bits of advice:
Familiarize yourself with some basic Excel formulas so you can concatenate and split information when needed. If you don't have a strategy on how to approach the task from point A (organizing the spreadsheet) to point B (executing the import command), then you may find yourself spending more time messing with the spreadsheet than if you had manually started with the IDs
Keep in mind that CSVDE does not create a password for the users that you import. By default, it'll create the users with blank passwords, and if your domain security policy prohibits blank passwords, then the accounts will be disabled by default. To set passwords for them, you'll need to use a VBScript or some other method to set the users' passwords.
If you want to see a list of all the available AD attributes, export your existing directory to a CSV file and isolate the user accounts. Keep in mind that although the exported CSV file will contain all attributes, not all of them can be imported with CSVDE (some area automatically created by AD)
There's also LDIFDE, but because of its format and difficulty of use, CSVDE is preferred for this kind of task.
