Dont get one.  I had one for over a year and I thought it was nice, but its soo small theres hardly anything to balance on, it doesnt grip due to small edge.  The chain wear thing is true my chain has strecthed, you dont get as many gear ratio posibilities.   
 
Im now on a 20t lathed and its much better, have something to rest on and it grips, also its not much heavier.  I wouldnt reccomend one from personal experience.