The difficulty with this question is that we have a sample size of 1 for the phenomena of 'life'. We do have many theories about how life came about, the RNA world hypothesis for one, but there are others too; the problem with all of them is that we still haven't been able to take theory and turn it into a successful experiment (i.e. no life has been made from any 'soup' of inorganic and organic molecules)...with that in mind, here is the way i see it
One thing that does stand out for me is that life began almost as soon as it could, probably about 3.8 billion years ago. This at least suggests that life, given the right conditions (habitable zone + water + organic molecules), can take hold with reasonable ease. There are probably a number of things that need to come together for life to begin; it's weather or not these conditions come together in the right way that may be uncommon...or perhaps a short timeframe for life to happen, if the conditions wait too long and the planet cools too quickly, then the necessary reactions might not take place.
So it may be easy for life to start, but all the necessary conditions, at the right time, in the right place, may be uncommon... Another thing, it also can't be too easy because, as far as we can tell, life only arose once here on Earth.
Or, maybe life takes hold rather easily, but the evolution of complex life is difficult; it did take about 1.5 billion years for eukaryotic cells to appear. Just so it's clear, that is a VERY long time!! Consider how bacteria multiply and evolve, think of how many generations and genetic permutations these microbes must have gone through before the right cells came together.
All in all, I think that life, especially complex life, to begin is probably a rare event...but it really is difficult to say......