Hiya Box.
This is a debate that continues, some guys hate them, but most havn't tried them!
I use lash clips now and again, and are really useful in the right situation. Very large format, rectified porcelain, especially polished and these babys are really handy. Smaller tiles (less than 600x600) and the system is expensive and a pointless.
These won't get you out of trouble on a poorly prepared floor, that needs to be right as with any job, they are no quicker and a bit messier.
As with any floor you will need to use the correct bed of adhesive. But they will give you the edge especially if you have polished porcelain on a large floor with lots of windows, the reflections on the rtiles show up any lips, the lash clips will help.
Tradetiler sell the Lash Clip system at as good a price as i've seen. Remember you don't need to buy the same amount of wedges as these are reusable once the tiles have set.