Jacques Brel is indeed amazing. Funny you bring him up since many people like to compare Stromae (who was mentioned a few pages back) to him. Mostly because they have a similar accent (both Belgian) but Stromae does have Brel-ish mannerisms on a few of his songs.
As for the accordeon, it certainly doesn't feel underused to me, but that's because a lot of the French indie bands I like use it a lot.
Here's my favorite Brel song by the way :