Andrew,
I have a couple of old Dolphins which have the heavy canvas-like bags, which can pick up everything... leaves, little pebbles, and dust. I did not find it hard to clean the bags at all, takes five minutes, but you will get wet.
I now have a year-old Dolphin Premier, which gives me three options....
1. A thin, cheap-feeling, bag which can be used for leaves and big stuff, but leaves all the dust or dirt behind... OR... 2. A fine screen mesh, which picks up small pebbles, but not much big stuff or any dust or dirt... OR... 3. A pleated flat cartridge filter that picks up sand, dust, and dirt great, but also captures small pebbles in-between the pleats. I find cleaning them to be a PIA...
My next robot will be the S300 that diverminh is recommending. I like the fact that it uses a "bucket" that can capture leaves and large stuff, a mesh "pre-filter" to capture pebbles, sand etc, and a fine filter to capture dirt and dust, all at the same time.
Jim R.