I use both a 3D touch probe and touch plate i.e. both of the methods Jazz describes. I have a mini toggle switch rigged up to switch between the two circuits, but it does mean going into Mach3 to change from active low to active high depending on which one I am using.

I would say the 3D probe is more convenient once its in the tool collet as my machine laying flat means I would need arms 6 feet long to click the probe button and hold the plate to the part. That said I have had to use the touch plate only and it works fine as its very simple and I still use it to set my tool height.

The 3D probe also gives you the option of "scanning" objects into MACH3/CAD (or it would if I find time to work out how ;-) ).