I thought having Navigation in my Z4 was cool, but I honestly never used it much. As a result it wasnt a factor when I was looking for an E92. Needless to say, I purchased an E92 without Navigation and I have no regrets.
In my opinion it is one less electrical device to go faulty = more piece of mind for me.
'11 BMW 335i M-Sport Coupe xDrive [LeMans Blue] - COBB AP V3, BMS Intake, ER CP, Exotics Tuning CF Lip, CF Diffuser, M3 Spoiler, Eibach Springs/Bilstein Shocks
'07 BMW 335i Coupe & BMW Z4 3.0i