Depending on what you're using the M-Path for, it can be pretty great or kind of craptastic. If you're looking for a relatively safe, direct bike route to get from point A to point B along US 1, then the M-Path is a great option for you. If you're looking for a jogging or bike trail for some serious workouts, you will be disappointed in the M-Path.
Here's why it sucks for leisure and exercise: It crosses many busy intersections, you may "lose" the trail in one or two sections due to poor signage (but I didn't have quite the problem with this aspect that others did), south of the South Dadeland metrorail stop it gets pretty sketchy (i.e. Don't go there after dark. Ever.), lots of bums doing their bum-thing along the trail even in the fancy areas of Coral Gables, you're right on US1 so you're pumping your lungs full of car exhaust, cars honking, running through the metrorail stations because that is where the path goes.
Here's why it's not so bad:
Every single intersection I've come across has been well-marked and (mostly) safe (if you obey the law when it comes to yielding to traffic). Also, the pedestrian walk/don't walk lights are automatic, so I like that you don't have to push each one and wait for it. The trail is very wide and generally well-maintained. Surprisingly, drivers are slightly more cognizant of pedestrians along this trail than in most other parts of Miami. The areas north of the South Dadeland metrorail station are relatively pretty (for being along a major roadway) and seem quite safe. Using the trail very early in the morning (or perhaps very late at night) will allow you to avoid a lot of the car-related issues and the awkwardness of running/biking through the metrorail stations. Even if you lose the trail for a quarter mile or so, it is pretty easy to pick up again after you get around whatever obstruction it was that diverted the trail (like the South Dadeland metrorail station).