Many of our customers are teachers, students, and hobbyists, but we do not get a lot of visibility into what they are actually doing with our products. What I can say is that the 3pi has done well in our local competitions, and we have heard of people winning various other competitions with it. Generally speaking, I do not know of any similar pre-made robots or robot kits that can outperform the 3pi in tasks like maze solving and line following, but the 3pi is not specifically optimized for these applications, and I suspect someone with suitable experience could make a better custom robot. The main point of the 3pi is to provide a highly capable robot so that people can focus on the challenge of robotics programming without also having to work out a good mechanical and electrical design.
In case you haven't seen them, we have a number of videos showing what we and some of our customers have done with the 3p:
You also might be interested in this video of a looped maze competition we held here at Pololu a few years ago; most of the entrants were based on the 3pi:
By the way, you might also find our newer Zumo 32U4 robot interesting: