Here are some of the basic comments that have been made about the Sonnets over the years and decades since the 19th century:

PERSONAL

“Scorn not the Sonnet … With this key, Shakespeare unlock’d his heart.”
                                                                      William Wordsworth, 1827

INSIGHTFUL

"The Sonnets on the whole contain such a quality of thought as must astonish every reflecting reader."

                                                                       Alexander Dyce, 1832

FACTUAL

“The Sonnets as a whole are concerned with actual fact.”

                                                                       Thomas Tyler, 1890

SPIRITUAL

"Whoever wrote the Sonnets must have known the depths of spiritual suffering."

                                                                        Barrett Wendall, 1894

CHRONOLOGICAL

“The analogy of a correspondence, carried on over years between friends, offers the best clue to their varying continuity.  Their numbers seem to have been chronologically arranged.”

                                                                        George Wyndham, 1898

LETTERS 

"There is much to support the view which holds that the Sonnets are a series of verse letters written to two people on the subject of the poet's relation to them." 
               
                                                                        Edward Hubler, 1952

AUTHORIAL

"It is a reasonable assumption that Sonnets 1 through 126 are in sequence.  There is a logic and rightness in their order which is greatly superior to that of any proposed rearrangement ... and this order is at least as likely to be the author's as the editor's."

                                                                         Northrop Frye, 1962

CONFESSIONAL

“What is astonishing about the Sonnets, especially when one remembers the age in which they were written, is the impression they make of naked autobiographical confession." 

                                                                          W. H. Auden, 1964
  
AUTOBIOGRAPHICAL

“There is nothing else quite like them in our literature, and none that have made so ineffaceable an impression upon men’s minds.  This is the reason: they are not ‘literary’ sonnets, in the way that so many sonnet sequences were … They were intensely autobiographical.”

                                                                          A.  L. Rowse, 1964
 
 PRIVATE DIARY

"He wrote them, I am quite certain, as one writes a diary, for himself alone, with no thought of a public.  When the sonnets are really obscure, they are obscure in the way that a diary can be, in which the writer does not bother to explain references which are obvious to him, but an outsider cannot know."
   
                                                                              W. H. Auden, 1964

CONNECTED

“The continuities are often self-evident … A rhythm, a rhyme, a quirk of syntax, or an echoing image: such minutiae, hardly discernible in conscious reading, knit the poems together … Links like these – and they recur throughout the sequence, with particular density in Sonnets 1-126 – suggest that the poems need no reordering.”

                                                                             John Kerrigan, 1986

THE PROBLEM

“The real problem of the Sonnets is to find out who 'Shake-speare' was.  That done, it might be possible to make the crooked straight and the rough places plane - but not till then … It has sometimes been said that if we could only know who wrote the Sonnets we should know the true Shakespeare.”

                                                                             Sir George Greenwood, 1908

THE LANGUAGE

“This autobiography is written by a foreign man in a foreign tongue, which can never be translated.”

                                                                             T. S. Eliot, 1927

THE TIME FRAME
"The question WHEN the sonnets were written is in many respects the most important of all the unanswerable questions they pose.  If it could be answered definitely and finally, there might be some chance of establishing to general satisfaction the identity of the friend, the dark woman and the rival poet (supposing that all were real individuals), of deciding what contemporary sources Shakespeare did or did not use, and even of determining whether the order is the author’s or not.  In the past and at the present, such a solution has been and remains an idle dream.”

                                                                             Hyder E. Rollins, 1944

THE PUZZLE

“The Sonnets of Shakespeare offer us the greatest puzzle in the history of English literature.”

                                                                             A. L. Rowse, 1964.