PIBS are paying a very good rate of return at the moment BUT this is because of the bank crisis. If a Building Society fails PIBS holders are last in line for a payout. In the past there was little chance of this happening but now we have a whole new "ballgame" and as the perceived risk grows the value of the PIBS falls and this makes the return increase.
If you look at current return tables (find them using Google) and compare, say, Nationwide with Halifax you'll see what I mean.......
If you have confidence in a particular Building Society and it's confirmed by the market (ie the return is relatively low) then they could be a very good investment because as the market gains confidence in the banking sector you could make a good capital gain as well a good interest rate....
This is obviously opinion and not advice,,,,,,