(The article below first appeared the 18th March 2009 on the hegel.net Hegel mailing list. The 2020 article below is an edited and expanded version (I mainly corrected spelling errors and added links and footnotes).
It was first planned that Eduard Gans (who was together with Marheineke the de facto leader of the Berlin Hegelians after Hegel’s death) was to write this Hegel Biography. Only after Gans died so early, Rosenkranz was chosen. Rosenkranz was not among the original list of publishers of Hegel’s work, and it was mostly because he had enough time that he was chosen (for Hegel’s wife, it was also important that Rosenkranz made a polite and not radical impression, so would not bring Hegel into a bad light in his biography). So officially selected by the Hegel family and the editors, Rosenkranz could get full support, including full access to Hegel’s letters and notes.
Many of these have been lost since that time (in 19th century, people had the habit of asking for original writings of people they honoured, just like today they ask for signatures, and so many papers were given away that way, also the family gave a lot of these Hegel papers which they considered unimportant to a paper mill1). So what Rosenkranz tells us out of these papers lost is the only account we have of the content of these papers and so in this respect, his biography is unsurpassable. So you will find that about 90% of the content of ordinary Hegel biographies is taken from Rosenkranz biography and as well as from the collection of letters, personal papers and remarks of other people on Hegel published at Meiner Verlag, Hamburg.
Haym’s biography was the second biography to be published on Hegel, and it was done at a time when the memory of Hegel was almost lost. In that biography, he published some material not mentioned in Rosenkranz’ work, but it has been mainly lasted as the single most important source of bad mouthing Hegel in the German history of philosophy as well as public opinion (its approach to Hegel is similar in tendency to Russel and Popper).
BTW, Haym (later a middle-right liberal) was a student of right Hegelian Erdmann in Halle. According to Erdmann’s biograph Glockner2, Erdmann was very proud in the freedom of science and so wanted to motivate his students to only study for truth, not for authorities by granting them that they would pass his exams whatever they say. Haym successfully tested Erdmann by passing Erdmann’s exams without knowing anything.
Regarding Schleiermacher, it is true that Rosenkranz published a critique of Schleiermacher 1836. However, Rosenkranz, like most students of Berlin University studied also at Schleiermacher and was - again like most students (including Hegelians) of his time - influenced by Schleiermacher. So the general picture, that Hegelians were just enemies of Schleiermacher is not completely true.
Judging from the biographies of the time and the reports from the Berlin university I read , beside studying under the professors of the Hegel school, a student interested in Hegel’s philosophy in the Berlin of the 1820’s and 1830’s would also hear lectures especially from Boeckh (Greek Philology), Ritter (Geography), Schleiermacher (Religion), Savigny (Law) and others (for example Marx heard lectures on the history of law both from Eduard Gans and from Hegel’s enemy Savigny).