I've got one of these
steerer extenders on my current commuter bike.
Since it works like a quill stem the internal diameter of the fork steerer tube is important. Mine works fine on my steel forks but won't fit in thicker alu steerer tubes or possibly other steel steerer tubes.
Since it just extends the steerer tube the spacers and stem cover it up so it looks like you just have a fork with a longer steerer. Mine has coped fine with mild off roading but only extends the steeerer about 35mm. I'd feel a bit uncomfortable bashing it about a lot at full extension.