Someone around here some time ago showed a rating chart, Worf's inclusion didn't help like they thought it would, but it didn't hurt ratings either.
I have read (can't think of where, though) that Dorn brought a lot ot levity and fun to the set, the kind of atmosphere that the TNG cast had. Before that the DS9 cast was very "by the book."
Worf's inclusion is what guaranteed I would start watching.
I don't even think Worf was written very well until later in his stay. In all his season four episodes he seems like a petty, dangerously impulsive manchild. They started to figure out how to write him better in season five.
I agree. I also came over to DS9 with Worf and I wasn't crazy about how he was written in that 4th season. His personality seemed very different than we'd last seen him in TNG S7.