To what extent is reviewing work beneficial for young researcher's career?
I judge my review opportunities based on the venue: if it is a known good venue, or if it is a little-known venue but there are good people organizing, then it is a good place to give service.  You might end up with all bad papers (after all, in a selective conference most papers are rejected), but you'll still learn things about the process and the way that other people are thinking about the field, and you'll give service that will help a venue or people that you like (and it's not bad on your CV).

Don't waste your time on junk venues: if a venue is not serious, you are only degrading yourself by associating with it.  Note, however, that "junk" is completely different than "low impact" or "high acceptance rate"---in many areas of computer science, a lot of the really interesting new work and research discussion originates in workshops or symposia that have very low impact and high acceptance rates (consider, for example, the AAAI Symposia).  What is important is whether the people and organizations involved are credible and likely to attract interesting work.