The best thing about ADX indicator is not even the di+ and di- lines... its the ADXline itself.
ADXLine indicates the strength of the trend, but NOT direction. Generally if ADX value is above 25, trend is strong. That strong trend could either be bearish or bullish.
Also if adx line is moving down from, say, 50 to 30.. that doesnt mean trend is getting weeker. It just means that trend is getting 'less stronger' (difference in meaning)
ADX is probably one the best indicators if used correctly.