+ 1 using their devices since a long time and know the founders. It's stable and nice, yes the hardware of tobii is a bit better, yet the extensibility of tobii is terrible.

Pupil Labs have their code on github (you can also use it with none pupillabs cameras :). It's python and easy to change /extend. here's the repo: https://github.com/Pupil-Labs/Pupil

Check also the branches and who forked it ... there are a lot of experimental eyetracking and other algs implemented.

We already used them for a lot of research projects (implementing smooth pursuit interactions and similar).

Edit: added the github repo of the desktop software.