I've stayed at the hostel twice, once in 2007 and then again 2012 and I love it! They are involved with many great projects and are very helpful in finding one that's right for you, no matter what your plan is (a friend and I needed a project for our Uni studies and they were very helpful!).
The hostel has good food, laundry included and many wonderful people to hang out with. So it's easy to come and just enjoy your stay in Moshi.
I we've recommended several people to go and know that all of those who have has enjoyed their stay.