A few notes on bards.
1. Bards are (usually) a support class. This has been dealt with well enough by others. With inspirational boost, they really dramatically increase the damage output of a party.
1.5 What hasn't been discussed yet is inspire competence and inspire greatness. Inspire competence will often provide the bonus for a maxed out rogue to succeed at finding or disabling an equal CR trap while taking ten. Which means that a sneaky party can travel at twice the speed and much more security (assuming the inspiration can work through a message spell, rary's telepathic bond, or the bard has the subsonics feat).
Inspire greatness is very helpful as well--especially at high levels. Not only does it grant temporary hit dice for a bonus to hit and hit points that stack with almost everything, the bonus hit dice also make characters less vulnerable to a number of HD based effects. For instance, I was playing a Living Greyhawk interactive with my (the 14th level) character about a year ago. First round into the final battle, the enemy casts blasphemy. My character is completely unaffected. Why? Because the bard's inspiration made him count as 16 HD instead of 14. In another adventure, I wished our party had a bard to do that. Our 12th level party was beset by Hezrou who could keep us from doing anything almost indefinitely with their blasphemies. Had any one of us been inspired with greatness, it would have made a tremendous difference. (Since that character would have been able to act and possibly put up a silence spell or something similar to stop the blasphemies and enable the rest of the party to act).
2. Bards are probably the best class for an intrigue focussed game. With all of the social skills (bluff, diplomacy, sense motive, intimidate) on their class list a heavy selection of enchantment spells (often at reduced level vis a vis sor/wiz), the bardsong fascinate ability (which lets them oppose their skill check to the enemy's saves meaning it will usually succeed) and the suggestion ability, as well as exclusive spells like modify memory, a bard is the best choice for a spymaster, etc. type character.