It was discussed here: Trading Large Amounts Relative to Daily Volume – Strategy
Out of curiosity, I just had a look at the most typical S&P 500 ETF traded on SIX in USD at a time when NYSE is closed.
The spread is as low as it gets: barely noticeable and certainly not higher than VT’s or other large US-domicilied funds. I am confident that for mainstream products on regular business days, SIX works pretty well.
Happy to stick to CHF-traded products though 
