While I'm not sure that I'd call it a must have, it is very handy to have. Mine has stayed hooked on my stroller since the day I've gotten it, and I use it to hook up items that I don't want stuck in the basket down below. The rubber/foam wrapped around it tore within the first week, but it could have been user error rather than design flaw.
Originally I only intended to get the free first box and then cancel. However after having success with the staying power of the polish, I decided to stick around. Their colors are bright and fun, and when you can finally get through to their customer service (good luck), they are very friendly.