I had the chance to see that NP had those sexy beauties. I decided I would order 2 just to try em. Well, I have to say that after about a week of use, my backpack is dry and never leaked. It has no seal, so you don't have to worry about loosing that goddamn seal. At 5.95$, consider it an investment. It is the best shaker bottle I used by far!
Downside: All my friends want to take the spring inside and make it bounce on the floor. So hide it when you go to class
Conclusion: Buy tons, you won't be dissapointed!