Technically it has 36, there are three buttons at the top M1, M2 and M3... each macro key can have 3 macro's thanks to those buttons.
I have the original G15 - have to admit I never really found a use for Macro keys but as a keyboard it is still going strong after 7 or 8 years.
The LCD for the G15/G19 has also nice features. If you use Teamspeak or similar applications, there is an addin for it. You can then see on the display who joins the chatroom, who is talking, you can mute, etc... Many games also display information on the LCD.
I also enjoyed the backlit keyboard and the fact that you can change it to any colour you want.
Meanwhile I don't play those games anymore, hardly any, and the keyboard was too big for me. But my parents still use it for WoW and my mom does her own macros and scripts, she uses pretty mych all the Macro keys. She absolutely loves that keyboard.