We love these doors.  Actually purchased two of them to cover a larger opening between two rooms.  We ordered the 42" doors that came with rail extenders, but didn't need the extenders since we joined the rails to make one long track for both doors to slide back and forth on.  The doors do not come assembled, and assembly is a bit difficult.  The video posted on the product page is a must when you're ready to assemble and install.  We also got a nice piece of rough cedar and stained it to use as a mounting platform for the rails.  It looks awesome!  You'll definitely want a mounting platform for the doors, as it looks better and brings the doors out past any existing moulding, light switches, etc.  I would highly recommend these.