I have an 8GB Sd card in my 550. You can choose what portions of the country to load to memory and if you never ride east of St.Louis then you can probably load everything in memory. I say probably because of the variables it gives for loading maps I believe there is a Western US option. I also keep some MP3's on the SD card for the Zumo to playback when I want some music. I saw them available for $650 at quadratech.com . I purchased the lifetime updates because what I have noticed over the time I have used the 550 is that not all the roads are in the map and some roads are in the map that shouldn't be (they were roads once but now they have been closed and removed) and roads in the wrong place.They are also updating the maps with new businesses. I have used the 550 to travel out west and will be running out west again this year and to Alaska next year. I want all the updates I can get when it comes to the road data. I don't run interstates or highways much, I tend to travel state and county roads. I enjoy the proverbial small town America. The only time I jump the interstate is if I need to make some time.
Features I like best and what I like least. I have a hard time answering that because I just really like the unit. I can't really think of anything I dislike well there is one trivial thing, when the phone loses connection it pops up a dialog box on the 550 to let you know. I have missed a few turns because of that damned box and there is no way to disable it without disabling the 550 speech for directions. I think it is just a solid unit. I have no regrets with purchasing the unit.