I have a Garmin nav in our Jeep, and my wife and I HATE it. Its awful...I honestly don't see how anybody could like that abysmal system better than whats in the Lexus. Only thing we dislike about the Jeep.
Oh...and Jeep cuts out the destination input when driving too...even though its Garmin.
If you are disappointed with the quality of the Garmin navigation in your Jeep, have you considered OnStar FMV? It's compatible with a lot of vehicles and you can get turn by turn navigation with it, besides all of the other things that come with it, like automatic crash response, roadside assistance, and bluetooth. Might be something to look into.
