I played and beat this game before I played FF7 and I still liked it.
I really enjoyed Dirge of Cerberus. I bought this game a very long time ago, and beat it without even playing FF7.
Time went on and PS3 came out so I basically forgot about all the great games that were on the PS2. Eventually my PS3 broke, so I went back to playing older games. I decided to finally beat FF7, then I put back in Dirge of Cerberus for old times sake. This game is still awesome to this day. It was one of my favorite games of all time way back in the day, and still is. It is very easy to pick up and play, very simplistic game with controls. But if you don't like the original controller scheme, then you can fully reconfigure it to your liking. If you have never beat FF7 and don't want to play a long game before this game, you can read about the events of FF7 that lead into Dirge of Cerberus in the manual. When you are done with the story you can unlock side missions to play which are fun to do, but they have nothing to do with the actual story. I hope they do remake this game for the PS3, because I would love to replay it with updated graphics when I buy a new PS3. Or make another game just like this that lets you play Barret's story before he meet's Cloud.
You should give this game a shot if you already haven't.