How detailed should my review of a very poorly-written manuscript be? My question has to do with whether my responsibilities as a reviewer also includes copyediting duties.

I am in the midst of reviewing a manuscript for a mid-tier applied health sciences journal.  While the topic is potentially important, the manuscript is poorly written and the methods used were simply inadequate to answer the research question.  The manuscript read like a first draft, rather than something that has undergone some degree of polishing and internal review before being sent out to the journal.  Given a manuscript laden with spelling and grammatical errors, awkward sentence construction, and incomplete ideas, how detailed should reviews be for very bad manuscripts?  Is it my job to "copyedit" (correct the grammar, spelling, etc.) the manuscript, or should my constructive criticism be focused on improving the big picture issues, such as:


Whether the authors convinced me that the study was timely or necessary. (No.)  
Whether the methods were appropriate for the research question. (No.)  
Whether the authors' conclusions were supported by the data they presented. (No.)


My annotated hard copy of the manuscript is full of comments in the margins about spelling errors and awkward sentences, which can be found every 2 or 3 sentences. I am of course more than happy to put in the review, but at the same time, I do not want to "embarrass" the authors by essentially saying that they do not know how to write.

I am curious to hear others' experiences with bad manuscripts and how they have handled them.
I view my job as a reviewer to be about triage and the "first order" issues with the manuscript.  Thus, for a very good manuscript, I will point out minor copy-editing issues, because those are the first-order issues remaining.  For a not-so-good manuscript, I will note if the manuscript needs language editing but focus on the scientific issues instead, because any specifics of copyediting are likely to be obsoleted in any case by the larger repairs that are necessary.