Not good... It affects the style of your forum, it does not look nice... I suggest you place a smaller banner but over the whole length of the forum, or a 780x15 bar between a post (like dp forums)... You must place your adsense so it does not hurt your forum layout, but still has a perfect spot
it is important that place a ad in such way that people feel is natural, donot use borders, place in middle , than i think it will be a better