Hi Steve,
Where do you mount the laptop when trying to use it in the car? The biggest problem for me in trying to use my laptop for in-vehicle navigation is that there's no place I can mount it for effective use. It makes no difference what software I use because I can't put the laptop where I can see it or reach it safely.
For that reason alone, I still use a personal navigation device that mounts nicely on the dash where it does not affect my view of the road but I can see it without turning my head or even taking my eyes off the road. It's also quick and easy to stash in the glove box or my wife's purse when we're out of the vehicle.
So, there are two issues with using a laptop for navigation. One is, of course, the best software. That's largely a matter of personal preference. It would help if you could tell us what you don't like or what you feel is missing about Streets&Trips for navigation use so we could point you at something that solves those issues for you.
The other is mounting, which is another whole can of worms. Knowing what kind of laptop you have, what kind of vehicle you are trying to use the laptop in and what sort of mounting you have tried would help us suggest solutions that would be most suitable to your situation.
...ken...