The lady at our local quilt shop had one of these and when I was taking a class there, she talked about smacking lumpy seams with it. Then she said, they make these for quilters but they are kind of expensive. So just go to Lowe's and buy a rubber mallet but make sure that the rubber part isn't the black rubber -- get the grayish one because it doesn't leave marks.
That's what I did -- got my mallet at Lowe's for about $3.