Btoom! may have taken some ideas from Battle Royale in the sense that it's about a few people being transported into an island where the contestants have to kill each other to survive, however it is slightly different becaus the rules set are the same as those found in the popular online game Btoom.
Personally, I don't think it's a rip-off of Battle Royale because it is quite different. Battle Royale is still greater because it is more emotional and it is not ecchi, however Btoom! is a decent manga and deals pretty well with the problems dealt with the characters. Characterization is good so far, but we'll see whether we'll find more interesting characters.
I find that the main protagonist is annoyingly perfect the sense that he's not obese and a loser despite the fact that he's only interested in online games and is terribly smart, however I suppose that's probably what the mangaka aspires to be.