I feel that any stock should be sold if
- It is overvalued.
- There is better alternative.
According to the site if you select "Conservative" in the Assumption field the "Intrinsic Value" for the B shares is 4829 USD. This is above todays (19 sep 08) price of 4595 USD. So we still have some way to go before I feel I should sell. I would sell when it is well above intrinsic value and maybe closer to the Optimistic scenario price of 6500 USD.
As for point 2, Are there equivalent or similar businesses that are available cheaper? To answer that I need to research Leucadia National Corp and Markel Corp. Both these companies are similar to Berkshire. They have insurance operations and use the premiums they receive to invest in diverse businesses. All these companies have well regarded managements and are followed by value investors.
Thats all for now...