I fixed my mortgage for five years (2006- the present, at 4.85%) and I greatly regret doing so now. I missed out on the many reductions in interest rates that happened in the meantime. I made some small gains at the beginning of the term and again at the end but overall I did worse on the fixed rate that I would have done if I had stayed on the variable rate.
Be aware that when the banks set their fixed rates they are gambling (against you) on future changes in rates. However, they usually have a much better insight into future trends than their customers would have. For example, 4.85% seemed attractive in 2006 but two years later it did not. It's possible that at at that time the banks were able to predict a series of cuts in rates whereas I was not.
Also, if you fix, you are tied into the fixed rate by contract and if you want to break out of that fixed rate at any time you will be charged a penalty fee. This penalty fee usually makes it uneconomical to break the contract as they usually charge you roughly the amount that you would save by switching rates.
Five years is a long time to be on one interest rate without the option to switch, as I know to my cost. I would never fix again for longer than two years as anything can happen in the crazy world of mortgage interest rates.