My husband and I sat down to go through the household budget last night and discuss fixing. We've decided to fix for 4 years at 3.57% (currently on AIB standard variable of 2.25%).
We thought about all the arguments set out on this thread, and considered saving the difference instead, but ultimately decided that 3.57% is a rock bottom rate. We would have taken your hand off for it at the beginning of last year. I think rates will start to come up by the beginning of 2010. We have a very big mortgage, and my husband has had a 10% salary cut. If rates go above 4.2% we are in trouble. We decided to take the pain now for some security down the line. Our broker agrees and doesn't make a cent from our decision by the way, as we are staying with the same bank!
I think it really depends on your personal financial circumstances.