Cornerstones are in my mind a great tool for an aspiring author and well worth the money as my greatest frustration with all the rejections from agents was that they don't give you any feedback at all! Nothing, nada, niente! So you don't know if it's the story they don't like, whether they think your writing style is bad, whether its the grammar that's bothering them, spellings, pace... you get the idea! You don't even know if they bothered to read past the first page!
Cornerstones fills this gap. You basically send them your manuscript (or part of it) and for a fee they send it on to an editor who reads every single line of it and then writes a full report (different types depending on what you pay for) telling you what works, what doesn't, and what needs changing.
I went for an indepth report and received a full 16page typed report, giving a general overview and then breaking my manuscript down and highlighting specific issues. It was a revelation! Firstly I didn't realise I'd put so many cliches in it - which all had to go! They helped me with the pace and showed enough interest to give me the boost I needed to carry on.
It was like doing a mini creative writing course, and probably cost a similar amount, but it was all tailored to my manuscript.
I now had weeks of re-reading and editing ahead of me!