JRG,
The 1270 is an improvement over the other 1200 series machines but it is not perfect, no detector is. You will dig some deeper iron that fools all machines. Horseshoes, thick iron and even on occassion some rusty barb wire (with barbs attached) can sound good. Fortunately most of the bigger stuff can be ID'd by the size of the signal.
Now it is better than the 1236 and 1266 in that it will not have you digging as many of those really deep <EM>small</EM> rusting away nails.
The audio is modulated but only on the deeper stuff and some of the deeper signals on non-ferrous can sound like partially rejected iron until you remove an inch or 2 of soil to let the machine get a better read. Now I'm talking some really deep stuff not the 6-8" mid range targets.
Hope this helps and let us know if you have any other questions.
HH, Tom