If its been monthly since the start then yes, its been out almost 6 years. I think that it's slowly gaining popularity. I didn't like it at first. There is going to be a 666 satan anime starting at the end of the year, and that's always a good sign.
edit: i looked it up, it started in 2002. It's created by Seishi Kishimoto, the younger twin brother of Masashi Kishimoto, creator of Naruto, so I really wouldn't be surprised if it were picked up by more people in the future.