a excellent device for marathon training . I be run my first marathon in October and in a sort of weird way , this device be key to keep I on target . I use it for the entire week , against my set training plan -lrb- 32 mile this week -rrb- without charge it . at the end of the week , I pop it in the dock to download datum and recharge . I use it primarily to track my mileage , time , and pace . I also look at heart rate and elevation to determine improvement . the real beauty of this device be the ability to run ANY route you want , without even think about the distance before hand . the garmin Training Center software be so-so . I would really like to do more with the datum in term of analysis and graphing , but it be a pain with the raw datum -lrb- export function -rrb- . I understand you can use the web software -lrb- motionbased or connect -rrb- to do more , but I have not experiment yet with the web software . as for size , some have complain about it , BUT I do not have any problem with the size . I do not have big wrist either . it be certainly larger than a watch and it do take up a good portion of the top and side of the wrist . look at it this way , you look like a serious runner with it ! it \* may \* be a problem for a petite female with very small wrist . the heart monitor be also no big deal . I wear it all the time , and barely know it be there . sometimes when I sweat a lot , it start to slip down a little . you can either tighten it or occasionally re-adjust as you run . I do not like it tight , to ease my breathing . as for reliability ... no issue . Works flawlessly . it do take a couple of minute to acquire satellite signal prior to run . I be glad I do not buy the newer 405 base on its review . -lrb- but the new 310xt look very nice -rrb- in my opinion , you can not go wrong with the 305 .