My background in Chinese learning has been very varied, from learning as a child at Chinese sunday school and learning for a year at university to not practicing or using it for years. I came to China to improve my Chinese, and at the same time work. Unfortunately, my work is all in English, and I pretty much work 6 days a week for over 9 hours a day! So I have not had the opportunity to practice my Chinese as I had hoped. Fortunately for me, Purple Bamboo offer an extremely flexible schedule not only for when you have lessons, but also for your language level (as various other reviewers have already explained), so I am able to have evening classes from 7:30pm until 9:10pm.
I've now completed one set of lessons and paid for another, they do not take summer holiday breaks so you can keep studying over the summer! My Chinese has definitely improved with the Purple Bamboo and I highly recommend them to anyone looking for not only flexible learning material but also a flexible time schedule.
Anyway, enough rambling, the other review here is far more detailed than mine is and has all the information you need to know to make the best decision for you. Safe travels!