The one big advantage of cannon when it came to sengoku siegecraft was the effect it could have on morale. The best known case was the Osaka Winter Campaign (1614), where Ieyasu, who had amassed quite a large number of guns, blasted the castle relentlessly. This is supposed to have prompted Toyotomi Hideyori into talking peace (with some additional prompting from his mother, who was herself much agitated by the bombardment). Osaka had had cannon of its own, but these were outranged and out-positioned by the Tokugawa batteries.
But, as Anssi says, for the most part, cannon had a status somewhere between novelty and limited utility for most of the sengoku period, with occasional exceptions, especially in the western provinces. In addition, the records are sometimes vague on what constituted a 'cannon' and at times the word seems to have been applied to large 'wall rifles' (there's a better term, but it isn't coming to me at the moment...). One of the factors behind the relative scarcity of cannon was the difficulty the Japanese experienced in making their own. While they took to building arquebuses like pros, the casting of cannon barrels was another matter.