Love these, especially the fact that you can pop out the roller.  Probably the pop-out feature is for cleaning, and that does help, but I like to pop out the roller and wrap twine or rubber bands around it to roll in ink/paint and make interesting linear features in artwork.  Works great so I bought a second one because I got the first one kinda icky after neglecting to clean it properly.  Still works for some things (like if I want an irregular surface).  So now I have one for smooth surfaces, too, which, I'm sure, is the original intent.