Almost all non-pro enthusiasts never sharpen the edges - ever. But any pro knows that a sharp tool makes for much faster work, especially in hard or compacted soils or soil with a lot of clay content.
After talking to several commercial experts, we have settled on a 10 bastard cut (i.e. relatively coarse) mill file made by the German firm Pferd. It has a molded handle, is the very best in class and is the overall best value. If you are an active gardener, you definitely should have one handy and use it regulary.