I have a couple different blender pens... I had always used the Dove blender pen as the tips are replaceable and they are refillable. I do find that the tips get frayed and scrubby pretty quickly... maybe I was using it too hard? I suppose you could use the refill solution and blend with a brush too... but then... if you're just working with something waterbased, you might as well use water and save your money.
A blender pen, though, can also be used with chalks and the fluid has a binder in it... I'm sure there are other applications for it that I'm not thinking of right now... haven't really used it much lately. I did recently get some Stampin Up blender pens from a SCS friend and they have a nice brush tip with a fine point which I've used with the Inktense pencils in tight spaces, like on the bicycle image... I also used Inktense and the SU blender on the grass in this swan card - had a little more control of the color where if I'd used water the lines would have bled.