There are some twenty thousand items there now, and the Darwin Online project director, John van Wyhe, has told Paul Sims of New Humanist that humankind may be "on the verge of a new revolution in the study and appreciation of the work of Charles Darwin".
The Darwin Online page announcing the papers' availability has this to say:
These online images are scans from copies of early black and white microfilms produced by the Cambridge University Library Imaging Service, mostly in the 1990s. For online publication now a slight colour tint has been added to many and the brightness and contrast have been digitally enhanced. Please note that many of the Darwin papers have been re-catalogued since microfilming in the 1990s. In some cases items have been re-ordered or moved to other volumes in the archive. In such cases the electronic images may not match the current arrangement of the papers in Cambridge University Library.
The immense value of this vast collection of material far exceeds the disadvantages of the occasional unreadable image and the lack of full colour. Several million pounds and years of production would be needed to produce colour digital images of the Darwin Archive. Hopefully this will eventually be achieved. In the meantime, most of the world's finest collection of Darwin's original manuscripts is now available for all to read, study and explore online and free of charge.
Read more here.