Hello William.Yao,
Please give some more details about these bugs? Until now I have been working with Nios and Nios II for about 1.5 years. And yes I encountered problems with some components. But I believe Xilinx has the same problems. And both companies are working hard for there customers to satisfy them. There are many advantages and disadvantages on both sides.
Beside the core itself someone who would like to use a soft-processor should think about the complete development environment. In my opinion Altera is about two years ahead. The Quartus package is perfected software and is very easy in use. The Xilinx ISE consists of a lot of small software modules which are not uniformly. The Altera Nios II IDE which is based on the Eclipse Project is really perfect development suite. All your needs are covered with Quartus and Nios II IDE.
I recommend downloading the Quartus II Web Edition and the Nios II Evaluation Version. Then start with a tutorial and get familiar with the stuff. A very good starting position is to buy a complete Nios II development kit. These ones are not cheap but each $ is a very good investment.
Regards,
cube007