It takes a little to get used to, but once you feel comfortable focusing manually, it's not too bad and for macro, actually preferable. The focal plane is so shallow, an autofocus lens will often spend too much time hunting for focus. If nothing else, its a good technique to get familiar with. If you have never used a macro lens before, I would suggest getting something on the longer end, like the 100mm, but it depends on what you think your subjects will be. The shorter focus distances allowed with a macro lens is a two edge sword. Its nice because its almost unlimited how close you can get, but that can become a problem, especially with a shorter lens like the 40mm. In order to get true 1:1 reproduction, (meaning a 1" subject in real life will consume 1" of the camera's sensor) the 40mm for example, you nearly need to be touching the subject. Very difficult to get enough light on the subject while so close and if its a subject that moves, like bugs, good luck getting that close unless they're dead. Even with the 100mm, to get true 1:1, I think I am about 12" away from the sensor. With the lens mounted and fully extended for 1:1 with the hood, its only like 3" from the front of the hood so it must be lit with a speedlight or off camera flash if its not a bright enough day out, (but thankfully, not everything needs to be shot at 1:1) . I personally like the 100mm range because its great as a portrait lens as well so its a double use lens. It's a great focal length to take to the garden or the park, whereas something shorter may be more useful for product photography in the studio.
Sorry so long winded, hope this helps.