I haven't linked MP up with my maggie 315 yet, but as far as the program itself:
It has lots of nice features, but the address database is NOT accurate. It confuses important differences (like So-and-so Street vs. So-and-so Avenue. And sometimes the numbering is just plain wrong. I don't know who they license their GIS data from, but it needs help.
It wasn't accurate in Texas (Ft. Worth area) and it isn't here in Tri-Cities, Washington either. I haven't used it in Seattle or Portland, but based on it's accuracy (misacuraccy?) in Ft. Worth, I'd be surprised if its any more accurate. (Well, maybe in Seattle....OK OK, maybe in Redmond....OK, I bet the Microsoft campus address is at least right)
The software itself is nice, but the database sucks.