Hi all -- I have been a member of gmatclub since 2011 and am happy to report that the MBA application process for me is finally over! I applied to HBS and GSB, was admitted to the latter (my first choice), and will join the class of 2016.
I used Sandy Kreisberg (the hbsguru / HBS guru / Cambridge Essay Service) and was extremely satisfied. I learned about his services after reading those Poets and Quants "My Chances" articles. Some people rip those as being sensationalist and views-grabbing, but looking back, there is actually a ton of insight there as long as you don't index too heavily on the percentages given (e.g., I think there was an early post that said McKinsey / Goldman Ivy-types with 3.8+ GPAs and 750+ GMATs got into HBS with 85+ certainty, which may have been true once, but is probably not anymore).
First, let me quickly advocate using consultants overall. You may believe that you have a solid grasp of your story and can write in a way that is clear, personal and convincing, but chances are, you're wrong. I worked in MBB for two years before applying and one drawback of the experience is it sapped all of my personal essay-writing ability. If you are in the same boat that I was in, showing essay drafts to your friends will not help as much as you think. They will ask you to clarify sections or fix grammar, but the problem is that they know you well and don't know the admissions process. You really need the exact opposite to cut through the BS and draw out your best stories.
Back to Sandy. I decided to work with Sandy for my Stanford app and used a one-time sanity check for my HBS app. The one consistent theme I read online about him is that he will kick your ass and does not sugarcoat his feedback. This is 100% true. My early drafts were bad and his comments were offensively direct. What I appreciated though, was that even when I sent in a bad draft, he was very specific about why certain paragraphs were ineffective. He never told me what to write, but after each draft I had a clearer picture of what my personal story was and what stories would best illustrate it. I went through eight drafts of the main "What matter most" essay before Sandy and I were both satisfied. I'm not sure that every consultant would have had the same patience and have maintained as high a standard (as opposed to saying after the third draft, "this is probably good enough, let's just edit it a bit and submit").
Let me also say that I thought that Sandy's style and level of service were ideal for me as an applicant. I had good stats (MBB / Ivy / 3.8 / 770), but because I knew so many classmates / colleagues that had gotten into HBS / GSB before, I was also at risk of thinking, "I can get in without great essays / recs / interview." In reality, every year there are many people with similar backgrounds that are rejected and it's getting more competitive every year. If you 1) have good stats, 2) are willing to put in the time to get into your dream school and 3) have the mental stamina to deal with round after round of harsh, critical feedback -- Sandy will maximize your chances of getting in. Don't underestimate the investment of time and emotional angst though. There were periods where I was really frustrated, not with him, but with the amount of effort I was putting in for what I saw as merely incremental improvements. In the end though, I learned a ton about myself and getting the call from Dean Bolton made it all worthwhile.