I too changed from a 550 to a 590. The screen is bigger, but I find the graphics particularly the low contrast pastel colour schemes harder to read in sunlight. Also, the unit has many features that I will never use - linking to smart phones, tyre pressure monitors and several jack and usb connectors on a lengthy cable that needs to be accommodated on the bike. Some of the text characters are smaller in comparison to the 550 and therefore harder to read if you are not a 22 year old American software engineer. The battery life drops off dramatically in a short period of time when trying to build routes by hand off the bike. The 550 display is back lit and can be brightened to the level one would expect of an IPad for example. The screen type on the 590 is higher definition but not back lit, dull in comparison to the 550. I understand that the 390 is of a different, back lit type and has fewer of the feature mentioned above. If I was choosing again, I think that I would think seriously about a 390 and save myself £200. I was seduced by the ever so slightly bigger screen and higher number - 590 has got to be better than 390, hasn't it? On the plus side, the processor is a good deal faster than the 550.
Alan R