I have the Mortise Pal but I really don't use it that much, dowels are so much faster and easier to do then making a mortise and tenons.
I guess I am not at the point of being a real craftsman and doing mortise and tenons, they are nice but the stuff I build the dowels have worked really well.
The Mortise Pal will do dowels but its not that good at it. The thing with the Jessem and the DowelMax is the way they set the reference surfaces. This makes both of them very accurate.
Now I really can not comment on the other systems like the Festool but then again all of these are less then half the cost.
Mortise Pal - $169, will do dowels but not that flexible in doing them
DowelMax - $310 will do everything with a dowel
Jessem - $199 will do everything with a dowel