If you didn't play any mod before, you will be surprised. The mods introduces you in a completely new and exciting universe. Most of them are so different, when compared to standard BTS, that you have to learn how to play again.
But, once you played certain mods, you will never come back to the standard game.
I have played, these last months, Rise of Mankind - A New Dawn 2 (ROM-AND2) and Realism:Invictus.