Well every stock in this market is worth investing provided you buy it at right price. All those people who have bought Renuka at levels of 30-40 can stay invested for long term. Becasuse these are probably rock bottom prices and God forbid if due to some extraordinary reason the prices still come down it would be easier to add more and bring your cost of purchase further.The purchases at current prices are likely to yield much greater profit in percentage terms than most of the stocks in Nifty if held for a long period and also downward risk is comparatively much lower.
I generally stay away from sugar and fertilizer stocks, since it is strictly controlled by the govt. All that I can say is that you should trade in these stocks only if you are a trader and it is advisable to keep your eyes glued to the price moment if you have bought any of these stocks.