Good review. I recently purchased M$ Streets & Trips also and I agree with most of what you have to say. I think you forgot to mention one con though (or i missed it). If you decide to take an emergency pit stop (gas, etc.) the voice navigation annoyingly lets you know that your off route in like 60 second intervals.
Other than that though this software is awesome IMO if you have a nice sized screen to use it on. I used it on my laptop and it was still a little hard to see while driving.
About iGuidance as compared to S&T's, it really depends on what you want to use them for and your preferences. I prefer the simplicity, large graphics, and unclutterd gui of iGuidance in day to day activities. But in long haul trips Streets and trips wins hands down. The number of configurable options such as being able to enter gas mileage so that S&T automatically calculates gas stops and schedules them on the route is simply amazing for those summer trips. (Dont be like me "Its okay wife, we'll make it to the next exit, huhhh, whats that noise) . But the same thing that makes S&T's great for longer drives makes it unusable for me for trying to figure out where the nearest ABC Company is in my city. Streets & Trips is layed out similiar to their office products meaning theres many menus to go thru sometimes to find simple things.