I use mine a lot! It is one of the few power tools which will cut flush, so if a dowel is sticking out of wood it will cut it completely flush without marking the surrounding timber. When the router has left the mortice with rounded corners the multitool is quicker than a chisel for getting them straight. The windows I have, have a sloped bottom rail which was great for single glazing but needs taking flat for double glazing, and there is no other tool which can do the job.
I will say though, like most tools corded is going to give much more power, and be faster and neater, so unless you really need cordless buy the highest power corded one you can.