I like to use one for broad pen work, especially for bigger pieces. It makes my work more consistent and puts me closer to it all over - I don't have to stand up to lean over and work on the top parts or hunch down for the bottom bits, you know? It's at close to a 45 degree angle for me too - I just love having that much vertical space to work with!
But it's a bit unwieldy, the board I have, and my desk is piled high with things that get in its way at the moment ... and also, I've been obsessing over copperplate for months, and I just don't use a slant board for copperplate at all. I have had two teachers actively recommend against it, insisting on flat for pointed pen work. I know a couple of calligraphers who prefer a very slight slope, but I don't even do that.