This may be a WikiTree G2G faux pas (my apologies to our inestimable moderators) but I'm going "answer" my own "question" just so that this post--because it contains a number of "for further reading" links--has its own little niche rather than being in the chain of comments. :-)
"How reliable do you think the use of imputation is?"
Well, it's been used for many years in SNP array testing, but as with all things that depend on DNA genotyping and interpretive algorithms, it really all depends on the size, validity, and thoroughness of the sample size, and on the accuracy and effectiveness of the algorithms used.
In this regard, in her blog Roberta makes a statement that may be a bit misleading: "Illumina has encouraged vendors to utilize the process called imputation to infer DNA results for their customers that are common in populations, but has not been directly tested in customer's DNA..." Some might read this as inferring imputation is something brand new with autosomal DNA. We have, in fact, been using forms of it for quite a while. A well know example is the BEAGLE routine that AncestryDNA uses. Too, if you look at the link Roberta provides to Illumina's explanation of imputation, you'll find that the datasheet is not new: it was written in 2013. There's even a post yesterday on Anthrogenica that states: "Until recently, the word imputation wasn't a part of the vocabulary of genetic genealogy..." So some incorrect assumptions are developing.
Debbie Kennett also blogged about the change to GSA on her site. She and Roberta both included links to this ISOGG chart that shows a comparison of the overlap of autosomal SNPs on the microarray chips in use by genealogy testing companies: https://isogg.org/wiki/Autosomal_SNP_comparison_chart. The new GSA chip is represented by 23andMe v5 and Living DNA.
I honestly have no basis to form an opinion on the reliability of the GSA chip. And despite the angle from which I view it, I have to recognize that genetic genealogy is not where Illumina makes its money, and is not near the top of its priority list. A quick look through Illumina's press releases tells us that. So it's little wonder that this news didn't make a bigger splash earlier, and that the genealogy testing companies likely carry little or no clout in the matter. Adapt or perish. And I'm certain they will all adapt.
What I do admit--in my uninformed opinion--is that the most troubling thing to me is the fractional overlap of SNP positions tested with OmniExpress compared to GSA. Comparing the GSA chip (23andMe v5) to the current AncestryDNA v2 product (OmniExpress chip), they overlap at only 149,394 SNPs, or 23.3%. With about 3 billion base pairs in a human genome, the 600K to 700K SNPs we've been testing represents only a minuscule fraction anyway. And if we reduce that fraction to an overlap of only 23%...well, I'm sure comparisons can be done; but my small brain can't fathom the statistical and mathematical hoops that will have to be jumped through to do it accurately.
And if we thought "matching" on very small segments was an iffy prospect before....
Well, fudgesickle. Looks like all the links made it run afoul of the 8,000 character limit. I'll try breaking this in two.
Edited 14 months later: Because this post came up on a search regarding Living DNA's new Axiom (Sirius) chip from Thermo Fisher Scientific, and I saw I'd written "12andMe" instead of "23andMe." Arrgh; sorry Anne. Not the first time I've done that, but normally I catch-and-correct.