Moment is an attachable lens that fits onto your smart phone. Moment has two different lenses, a wide angle and a telephoto. The lenses seem to improve image quality overall due to the high quality of the glass itself.
I'll have to admit, the images they show do look pretty good. Especially for a mobile phone with only decent camera quality, like an iPhone 5s. However, I know from experience with real DSLRs that combining lenses (in this case the iPhone native lens + the moment lens) never really actually improves image quality. Rather, it give you a different perspective because the glass is just different.
My suggestion is, if you can pick up Moment for a decent price I would give it a try. I wouldn't pay more than $100 for one.