What do the signal strength readings mean. What is a good reading and what is bad reading?

Cell Environment and RxLev Indication +CCED

The third to last number, or first negative number, gives the receive dB level. The range is from -107 to around -50, with -107db being the weakest signal, and -50db the strongest signal. A reading around the low -90's and -80s should be adequate, a reading in the -100's indicates a weak signal.

A possible response:
+CCED: 1,725,4,65535,6,,0,,,-104,-35,-63