Each bottle top is single use only. The nice thing about it is being able to build up vaccuum and leave it alone. I've said before that I can do close to 500ml in an hour.
If your doing small batches or just like syringe filters they are great, I just hate sitting there for hours lol. But then again I used .2 which is so damn slow.
Keeping things sterile is real easy. Once the media bottle is sterile you can just put the top on and be fine. If you ever make the jump I can explain how I like to sterilize/keep sterile and lay it out.