That sales number is good - but compared to the 9 million iPhone 5S / 5c sales that it achieved last year - with essentially nothing new to offer - is strikes me as the lowest number Apple could have hoped to achieve. If the iPhone 6 was going to tempt large numbers of Android users to switch (and that was clearly one of Apple's intentions) then it must have been expecting to sell many more iPhones than that.
Before we go to negative on those numbers though, let's consider some factors that may have impacted on those sales numbers.
The iPhone 6 launched in only ten countries, whereas the iPhone 5S / 5c launched in eleven - crucially the missing market is China. That alone could account for a smaller number than expected. However we know, from various videos floating around the web, that large numbers of Chinese nationals were in the queues to buy iPhones when stores opened on Friday. Seeing these phones being handed over for cash almost as soon as they were out of the shop suggests that a hefty black market in the iPhone 6 exists in China and many of those who want (and can afford) an iPhone 6 will have got one anyway.
Supply problems may have reduced the number of phones shipped over the weekend. There were particularly strong rumours that the 6 Plus had proved more popular than expected. Certainly the US Apple Store is quoting delays in shipping either version of the phone - 7-10 days for the 6 and 3-4 weeks for the 6 Plus, which would back up Tim Cook's assertion that they could have sold many more if the stock was available. However, I have to say, no-one I know in the US or the UK who wanted one has failed to get an iPhone 6 (although I don't know anyone who wanted the 6 Plus).
In the end, there are too many variables to make judgement calls. Apple sold ten million iPhone 6's last weekend. It's a good number, but I suspect there are many, both inside and outside of Apple who will feel a little disappointment that the biggest iPhone change ever hasn't led to greater sales.