According the BBC teaching assistants make significant contributions to
pupils’ learning.  Research  by London University’s Institute of
Education, said teaching assistants also faced a range of challenges.  The reseach was initiated because teaching assistants are now covering more classes with the introduction of PPA time.  The report said more work was needed on how much assistants were using
teachers’ lesson plans and on what pupils thought of them.

The BBC has some comments taken from the report:

• Assistants clearly believed that they made "significant
  contributions".
• They were acting as a bridge between teacher and pupil, interpreting and
  adapting what the teacher was doing so pupils’ learning was more successful,
  supporting groups and individuals, and promoting pupils’ autonomy.
• Teachers were "generally positive", welcoming the support and
  especially the flexibility that the presence of an additional adult gave them.
• Teachers and head teachers said assistants were "very valuable to them as
  resources and as support for their work".

Can’t fail to emphasis how much I agree with the last comment.  Well done TA’s!