They have their uses although i find them to be quite a hefty investment for the amount of use you will probably actually get out of one.
Alot of the time something can be found close by to substitute for a 3 meter ladder and its alot less dodgey using something you have found on site already. Their main limitation is the view the law would take on you bringing a ladder along to get in somewhere really, morally ok maybe but the law doesnt always follow common sense. Otherwise they are slow to erect and fold away, especially after they get full of grit!
Id say only buy one if you know of somewhere it would do a specific job. Maybe split the cost with a group of your mates and share the use of it. Thats what we do although in maybe 5 years of having it its been used like 3 times and one of them was for the decorating!