I have been using Showbie for quite a while, both at my school and with the schools I support. It has certainly solved the problem of how students can quickly send their work to the teacher and the teacher share resources with the pupils. We have used it a lot and we like how it can be accessed online on devices other than the iPad. It also works well for assessment and archiving work. The class codes can be given to the headteacher so he/she can see all pupil portfolios, making it good to show parents and inspectors.
Each pupil has their own login in normal use. However, some of what we do at school is collaborative. For example, a group of Year 6 pupils can each be working on a short video which could then be stitched together in iMovie etc. It is not possible to for one pupil to send a file to another using Showbie so we have created group logins. This means that a group of pupils would all log in using the same username and password. Each student can then submit a file to an assignment and the other pupils see and download that file onto their iPads. Importantly the teacher can also see whole groups work and each part of it.
There are many collaborative tools on the iPad but as many schools are using Showbie then this may create another possibility.