Hi Bob, I have the E70 which is the same as the M550 with regards to performance - same motors, brushes (including 1 active brush) and suction rate. Last summer was my first with the E70 and so far I am
pleased with it's performance.
The difference between the M550 and the E70 is bells and whistles. The E70 has a "Basket Full" indicator, more cleaning modes (e.g., "Wall-Only") and a temperature-sensor, which the M550 lacks. Check out the
Pelican Pool Dolphin Comparison sheet.
The "Ultra-Fine" filter is one feature that I would absolutely require on any electric robot cleaner. The E70 has a two-layer filter with one of the layers being the Ultra-Fine variety, wherease the M550 has only a single layer, but includes the Ultra-Fine filters. That would work for me.
Cheers!