Product review (3 stars): Beware - not a replacement lock
If you have an existing door with a typical doorknob lock, this is *not* designed to replace it.  I did manage to jury rig one of my doors replacing an existing doorknob with this lock but I would *not* recommend doing it.

This lock should be installed on a brand new door that doesn't have any existing holes cut in it.  To mount this lock properly you need to drill top and bottom holes (10 mm for the securing screws) and a main hole for the bolt (this is *smaller* than a typical doorknob hole!).

This lock is designed to be locked at all times.  There is a latch on the inside which you can slide which will keep it unlocked (however I believe this keeps it unlatched too (thus your door won't stay closed).

The instructions are weak, so if you aren't a professional you may have difficulty figuring out how to install it and getting everything "right".

Changing the combination is not a bad process but it does require dismounting from the door (which in my case I hope to never do!).  Although you can use some numbers twice, the sequence of the code does not matter.  Thus if your code is 1-2-3-4, anyone can enter by pressing 3-2-1-4 also!  Note: there is no number zero nor nine, it has only eight buttons for the code plus the "clear" button.

The handle can be easily switched for left or right operation.

If you can get past the hassle of installing it successfully, the lock does work *very* nicely.