I posted previously that I was going to use Safari's in-built Reading List feature in place of Instapaper for a little while to see how it stacked up and how useable it really is. After using it exclusively for a little while now I'm fairly impressed.
There's one big flaw: in iOS there is no API to add items to Reading List. Your only option is to open a link in Safari and add it via sharing pages. I hope that this will change in the future, and if you're a high volume user then this is likely to be a deal breaker. I'm not, and while occasionally irritating this didn't bother me as much as I had expected.
One benefit I hadn't foreseen is that Reading List items are automatically downloaded for offline reading where as in Instapaper the app has to be launched (or you actually need to have a net connection if you're using the website for the obvious reason). In normal use this isn't a benefit, I almost always have a net connection of one form or another. However once or twice it has been convenient to be able to access the articles while offline without needing to think about it in advance.
Reader view is also well integrated, and it remembers it's position most of the time if you browse away by following links in the article. It also only has to be toggled once, and persists between articles. Your mileage compared to the mobilisers of Instapaper or Pocket etc may vary, but I find it pretty ideal.
So I'm in no rush to switch back to Instapaper right now, and am going to give it a few more weeks before I make up my mind.