The gutter attachment is a "u" shape with two jets on the end facing both ways. All you do is hook it into the gutters and spray the cr**p everywhere!!
I suppose with any tool there are fors and against. At least you can clean them out from ground level, albeit if they are full you may have to clear up the mess afterwards (10mins), but then it's just something you allow in your pricing. If you are careful and obviously reduce the pressure for these jobs you can control the amount / distance the moss etc. falls away from the building, short sharp bursts of flow clear blockages without the mess going everywhere!
If you are going to go into cleaning gutters, i would look at other products, and ask or look on this forum for ideas, i have no idea about other product costs or if they are any better as i haven't used anything else. Sorry i can't help you more in that department. If you do go ahead, remember that if the building has a conservatory on the back, the roof will get dirty so allow time for rinsing it, or even better get it added onto the job as a clean!