It's easy once you do it a first time. My only mistake the first time was that I tried getting the upper clip off the hose without unscrewing the the hose off. I must have invented at least a few cuss words that day.
For as cheap and simple as a fuel filter is and the problems associated with some poor Focus owners, I'd change that sucker ever 15-20 k just for precaution.
I'd also run a bottle of Fuel Injector Cleaner every 6 months or so. You'd figure that if you did have some crud stuck inside your filters inside the fuel tank they'd hopefully break lose and let the larger filter catch them?