We've been using BIRT which had a steep learning curve for me until I realized how many WYSIWIG features it had (I started editing the xml source code direct, which I don't recommend.) There are some output specific tricks (like using a 0 left margin to not get a blank A column when outputting to XLS format) but for the most part it's quick and easy to use, edit and preview.
I have also been impressed on how easy it is to intermix different datasets in a single report. While not a silver bullet, its a better all around tool than 99.999% of people are going to build on their own.
