Simplicity. You had to signup and register for an API token for the others, and then read the docs to figure out how to parse the response or work out what field you wanted. Can't get much easier than:

    $ curl ipinfo.io
    {
      "ip": "73.222.35.113",
      "hostname": "c-73-222-35-113.hsd1.ca.comcast.net",
      "city": "Mountain View",
      "region": "California",
      "country": "US",
      "loc": "37.3885,-122.0741",
      "org": "AS7922 Comcast Cable Communications, LLC",
      "postal": "94041"
    }

Of course it's now evolved past that point. You can signup for an access token, we offer additional data sets (carrier detection, hosted domain details, ip type etc), and we've done a ton of work on infrastructure to handle over 300 million API req/day at very low latencies and without any stability issues, but simplicity and really focusing on developers is what I think helped with the initial growth.

> what do you think helped you fight through the crowd to become as big as you are now?

I call it the "Stackoverflow Effect" :) See => How to get client's IP address using JavaScript only https://stackoverflow.com/q/391979/325521

That question has over 1/2 MILLION views over an 8 year period, and is still very active almost every day.

Creator of ipinfo.io has posted his solution with link back somewhere in the middle / bottom, and still manages to get a lot of referrals from there. He wrote about it here at HN few weeks ago. Here's that thread : "How I Took an API Side Project to 250M Daily Requests (ipinfo.io)" => https://news.ycombinator.com/item?id=14678473