Subreview papers, to what extent? I am about to defend my PhD thesis in computer science and so far I have being asked to subreview 5 or 6 papers of different conferences.
At the beginning, it was exiting because it was a new experience in my career. However, at this moment I am a little bit snowed under work and I have being assigned with 2 subreviews from the same conference.

I do not want to turn down the opportunity of expanding my knowledge and experience, nevertheless, this situation has brought to me serious doubts about the usefulness of subreviewing.

Apart from the acquisition of knowledge and experience, 


Which are (or could be in a future) the benefits of subreviewing?
Is there a moment in your career that it is not worth it anymore?


Thank you.
(a) Benefits: You mentioned the biggest one, i.e. knowledge, exposure and an updated idea of trends in the field. Besides this, the major factor is generating/maintaining goodwill with the person who sent it to you for subreview. This is an intangible benefit which could multiply in ways that are hard to predict.

(b) When is it not worth it: Probably once you have defended your PhD. You may prefer to review directly. Definitely once you're in a faculty position, unless you do it as a favour to someone.