If you look at some previous dicussions on this subject, in general the outlook is positive on the navman kit,
Some things I have learnt is mounting is critical, use correct brackets so as not to distort casing. Map cartopgraphy can be an expensive add on, do you really need a map. Additional fuel monitoring is also a bonus, no other manufacture provides this option within a gps unit.
I have made my choice to purchase a Navman 500I + Fuel transponder unit based on advice given by users of the Ribnet.
There are better quality Gps units on the market but price also played a factor in my decision. As to life expectancy of the unit I consider a 2 year warrenty satisfactory on any kit.