What is considered good DSL Noise margin / SNR ?

Noise is a combination of unwanted interfering signal sources, such as crosstalk, radio frequency interference, distortion, etc.

Noise margin (signal-to-noise margin) measures the relative strength of the DSL signal-to-noise ratio (SNR).

Higher numbers repesent cleaner signals, with less noise. In some instances interleaving can help raise the noise margin to an acceptable level.

6dB or below noise margin is bad, it will experience no synch or intermittent synch problems
7dB-10dB is fair but does not leave much room for variances in conditions
11dB-20dB is good with little or no synch problems (if no large variation)
20dB-28dB is excellent
29dB or above is outstanding

Note that there may be short term bursts of noise that may drop the margin, but due to the sampling time of the management utility in your modem, will not necessarily show up in its interface.