Fit:-I have a tiny wrist, 6"", and I love this device.-I got Small/Regular, and it's a good fit when fastened in the 9th hole from the end of the band. I think there are 15 holes in all.-The device is light and the band is soft, so I no longer have to worry about it rotating around my wrist until the hard part of the watch is athwart my ulna, which hurts.-The button is on the front of the watch, not the side, so when I push the button it doesn't slide around my wrist.-There are many, many holes to use for fastening such that I can get a good fit.-The little loop that holds down the end of the strap has a little tongue that fits into the band holes so it doesn't slide off the band. It can take effort to get the band into it, but once it's there, it's really there.-I wish the hard part of the watch was shorter because it's wider than my wrist bones - if it were shorter, the fit would be better.-This fits much better than my old Vivofit 2, however, and I'm very happy.Function:-I had the Vivofit 2, and setting up the 4 was ridiculously easy. You don't replace the old watch, you add the new one to it. So for my first day I had the steps from my old watch from getting up until putting on the new watch recorded on Connect, then the new watch's steps were added to the daily total.-I think the step count may be more accurate - I know that the count starts showing up on the watch face sooner.-I love that the new watch shows both the time AND the number of steps at once - I don't have to choose. Scrolling back and forth was annoying with the 2.-Syncing seems easier.-The 4 did have to download some updates, I think, which took a couple of minutes.-I tried the ""find my phone"" feature, and it works great! It took maybe 30 seconds, but then my phone started making a pleasant sound that would be easy to find.-I love that I don't have to charge it, which means that I never have to take it off unless I just want to.Overall, this is a big improvement over the Vivofit 2.