I've been interested in the same question. It's hard to say because the genetics papers are very technical indeed, and the error rate is going to depend on how you define it, what kinds of edits you were trying to make, how you measure/detect errors (as well as how reliable your process is there), whether you're doing it to in vitro cells or embryos or whole organisms (in increasing order of difficulty), and, since CRISPR has been advancing so fast, when the work described in a paper was done (something done in 2014 and published in 2016 is going to be much less relevant than work done in 2016 but not yet published). For example, kbenson quotes a 60% figure, but that is for number of successful edits as opposed to non-edits or edits somewhere else; which one you care about is going to depend on your purpose.

For 'edits somewhere else', which are new mutations which might be harmful, as far as I can tell as an interested layman, in terms of the last category of errors, 'off-target' edits, the state of the CRISPR art is a low, effectively near zero mutation rate:

- "High-fidelity CRISPR-Cas9 nucleases with no detectable genome-wide off-target effects" Kleinstiver et al 2016, https://www.gwern.net/docs/genetics/2016-kleinstiver.pdf

- "Rationally engineered Cas9 nucleases with improved specificity", Slaymaker et al 2016 https://www.gwern.net/docs/genetics/2016-slaymaker.pdf

- Church, April 2016: "Indeed, the latest versions of gene-editing enzymes have zero detectable off-target activities." http://www.wsj.com/articles/should-heritable-gene-editing-be...

- Church, June 2016 "Church: In practice, when we introduced our first CRISPR in 2013,19 it was about 5% off target. In other words, CRISPR would edit five treated cells out of 100 in the wrong place in the genome. Now, we can get down to about one error per 6 trillion cells...Fahy: Just how efficient is CRISPR at editing targeted genes? Church: Without any particular tricks, you can get anywhere up to, on the high end, into the range of 50% to 80% or more of targeted genes actually getting edited in the intended way. Fahy: Why not 100%? Church: We don't really know, but over time, we're getting closer and closer to 100%, and I suspect that someday we will get to 100%. Fahy: Can you get a higher percentage of successful gene edits by dosing with CRISPR more than once? Church: Yes, but there are limits." http://www.lifeextension.com/Lpages/2016/CRISPR/index

In human embryos specifically, the published work is below state-of-the-art (unsurprising, given the taboo) but shows increasingly good performance in terms of making the desired edit and also not making undesired edits:

- Liang et al 2015 "CRISPR/Cas9-mediated gene editing in human tripronuclear zygotes" http://link.springer.com/article/10.1007/s13238-015-0153-5%2... http://www.nature.com/news/chinese-scientists-genetically-mo...

- Kang et al 2016, "Introducing precise genetic modifications into human 3PN embryos by CRISPR/Cas-mediated genome editing" https://www.gwern.net/docs/genetics/2016-kang.pdf http://www.nature.com/news/second-chinese-team-reports-gene-...

- Komor et al 2016, "Programmable editing of a target base in genomic DNA without double-stranded DNA cleavage" https://ase.tufts.edu/chemistry/kumar/jc/pdf/Liu_2016.pdf

- http://www.nature.com/news/chinese-scientists-to-pioneer-fir...

- "CRISPR/Cas9-mediated gene editing in human zygotes using Cas9 protein" Tang et al 2017 https://www.gwern.net/docs/genetics/2017-tang.pdf : no observed off-target mutations (but very small sample size); efficiency of 20%, 50%, and 100%