Here's a somewhat more well supported take on the bond market from Angry Bear than what you'll see most places. His model suggests that bonds are properly valued at the moment and were actually somewhat cheap not that long ago. I haven't looked much into his model yet to see if there's something I disagree with in there, but given levels of GDP growth and inflation, it could well be that present bond prices aren't that overvalued.
I don't happen to buy into the thesis that bonds are a bubble in the same way that stocks were in 1999 or housing was in 2006. I think they are only modestly overvalued on an absolute basis, but they are very unattractive to stocks at the present time. Let me put it bluntly. Do you think that the total return of bonds, which will be the present yields at best considering prices are likely to drop as interest rates rise, will exceed that of stocks over the next five or ten years? Even after adjusting for the relevant risk premiums, stocks are quite a bit more attractive than bonds at the moment.