Like the other reviewer, I recently upgraded to this watch from a 4 year old T3 and completely agree with his assessment. Instead of repeating, I'll add a few personal observations.
The M4 watch and its strap are both much better constructed and solid compared to the T3 set. While the T3 has the advantage of calculating and tracking Training Effect (TE), I love the simplicity of the M4: clearly delineated 3-button operation vs a more obtuse 5-button arrangement on the T3, large easy-to-read numbers and symbols vs smaller digits and scale around the circumference of the T3, and finally a very simple intuitive menu system. The newly designed strap also seems to hold in place better than the older version which had a tendency to slide down when I'm vertical.
Operationally, this HRM also tries to be encouraging with a notification that you are 50% through your workout, and at the end, "good workout!".
Price-wise, this HRM is hard to beat! Very highly recommended!