How much can you make by using playfire and is to worth it ?

Make what?

Totally worth it. Especially if you have quite a huge amount of backlog in your library.

Really worth it. But it's hard to get some rewards without hacking in such a short period. On this site i've already got 40$ in 3 months.

Mine's only been working for about a month, got £8, which is about $13 in that time. I could have had more, but I missed quite a few achievements that I couldn't be bothered trying for.

As has already been said, it's great for trying to tackle your backlog if you have one. I'm trying out loads of different games that were just sitting there doing nothing as a result of it, and discovering some that I like.