It is hard to determine if the Opium Wars were wholly negative or wholly positive, although the negative effects far outweigh the positive. On a positive note, the wars did open China to more trade with the West. The wars broke out because China had severely limited Western commerce within the country, and required that any Chinese goods purchased must be paid for in silver bullion. It was the lack of silver bullion which led the British to sell Opium in China for silver and use the silver for trade.
The negatives of the trade are, however, overwhelming. The British were aware of the addictive qualities of opium, but sold it anyway for purely commercial (if you will, selfish) reasons. When the Chinese understandably resisted, the British responded with superior weaponry. The Opium Wars ended with unequal treaties which crippled the Ming dynasty and opened China to domination by Western countries. It was the resistance to this western domination that later led to the Boxer Rebellion.
In sum, there was some minor positive effects of the Opium Wars; yet the overall result was overwhelmingly negative.