I used to have a Flex One and was always happy with it. I upgraded to an Alta after the battery died, simply because I wanted a display. But I heard that the new Flex is just as reliable, with some minor improvements (i.e. it won't go into sleep mode while pushing a shopping card over cobblestone).

What I liked about the Flex was how small and subtle it was, giving you various options to wear it without it being an obvious fitness gadget. Very easy and cheap to get unofficial accessories for, too.