I recently built a jack rest and it is pretty impressive step above bipods or bags (although I still use a rear bag).
It is the most solid shooting platform have used to date, these two videos are rifles sighted on target one before being fired (before being loaded in the first example) and still didn't move.
https://www.youtube.com/watch?v=ng39ce3Ii_w
https://www.youtube.com/watch?v=S1L4EJAcnhY