I quite enjoy Instagram. I don't have a teen yet, but my youngest SIL uses it a TON. I think 99% of her friend contact is through Instagram.
I like being able to share snapshots and attach a quick thought to them and I love seeing other people's photos.
It has the same risks as any social media, really. Anyone can message an account (I mostly get people trying to sell me crap). You can choose between private and public. Private requires you to approve followers for them to view anything but your profile picture. Public profiles can have inappropriate content, but if you stick with people/organizations that you know, it's usually pretty safe. I don't typically encounter anything bad because I just have my feed of people I actually know and a few companies and don't stray into trending or hashtags.