Spiral cutters have essentially zero tearout. They run quieter, and if you ever manage to knick one (which is many times more difficult to do than with a straight knife) you just rotate it around and you're back in business. The cutters last a very, very long time.
Spiral heads are expensive but require little upkeep and will produce a far superior finish. They're also far more forgiving if cutting across the grain. The shreds of wood will not clog a dust collector like the ribbons a straight knife will generate in those cases do.
Those are mostly positives that apply much more to planer use than jointers, and if I could only have it on a planer or jointer I'd pick the planer every time to have a Spiral head. But they're still nice, and if you do much or any work with particularly hard woods- for me, hickory, ash, occasional ipe or similar rock hard tropical wood- they're worth the price of admission. And I'll be honest with you, I simply HATE setting jointer knives, and I'm bad at it. Carbide spirals remove the problem.
Oh, and I also like that interlocked grain sapele that is so popular, and straight knives do unspeakable things to it.
