They are nice for when you need them but dedicated tools for anything is always preferable. What that is really good for is cutting in tight spots and undercuts at right angle you can't get rotary tool like a grinder or straight blade like a sawzall into too. During my renovation project so far there was probably one thing I could have used something like that for and it would have saved me about 2 hours, probably one of the few things I don't have in my arsenal.
If you're going to be cutting a lot of tile, stone, metal or whatever rotary tool can't be beat. Rent a machine or buy something like a decent but cheaper grinder. Be aware, a grinder will hurt you bad used improperly. Hold on tight.