I was seeing that they are testing new BOINC client versions. When they finish the testing process of one version, they start with the next one. Does anybody knows when it will appears a new finished BOINC client version?

I have been using 5.8.0 for the last few weeks with no problems (and if I find problems I report them).
As to when there will be a new "stable" version, there is still much work to do. There are at least two half-made features: the simple GUI, and the new tasks tab (which I haven't seen yet). New tasks list will have progress bars, and I don't know much more about it.