Pros
Three big factors for me: 1. They don't micromanage you. I've been tutoring for years and when I worked at once of the huge brand-name places, they control your every word. Next Step is more like an agency - they set you up with a student and then let you do your thing. It's a refreshing change from places where they seem to take a very "Big Brother" approach and watch your every move. 2. They deal with all of the headaches that you get as a freelancer. I've done a lot of freelance tutoring and the amount of b.s. you get is ridiculous, with clients showing up late, canceling appointments last minute, or giving up on the tutoring after just a few sessions because the going gets tough, or even worse stiffing you on the bill. Next Step gets the client to commit up front to dozens of hours of tutoring, and gets them to pay up front, so the work is much steadier than freelance. 3. BY FAR the most important thing - they don't lock you down with a non-compete agreement. When I was leaving the big brand name test prep place I called around to lots of different companies to start pulling some work together. And every place, even little podunk places, wanted me to sign a contract that said I wouldn't do any work on the side at all. Next Step was willing to let me continue to have my own clients, or drum up new work, and also bring me their students. That's what sealed it for me. Oh and one other - they've started doing more work with online tutoring and not having to commute in the congested NYC area is a huge plus.
Cons
The work can be sporadic. Some months they offer me more than I can handle, and other months are slow. That's not really their fault, I think, just the nature of test prep business. The pay rate can be lower than what you'll get if you're an experienced freelance (although it's WAY higher than what the brand name places give for starting teachers). But their hourly rate is absolutely worth it since it's headache-free work.