I have the 870 before the 875, the thread cutter is great when piecing, but not when machine quilting IMHO. As far as the auto tension never needing adjusting... that's true if you always use the 50 wt in top & bottom, but you have to manually adjust it if you have different weights in top & bottom.
Here's some other postings on the board if you want to check them out as well.
http://www.quiltingboard.com/search....apphire&u=&s=0