We stayed at Carien's flat for 10 days at the end of August for a holiday and a work conference and found it excellent as a base for both.
Hannah's very detailed review (May 2017) is entirely accurate. The flat is in the medieval centre of this historic centre and everything in the town centre is walkable (though you can hire a bike if you prefer!). Car traffic in the city centre is very light, and there is a good bus service too. The rail station is a short walk - nearer than it looks on (SENSITIVE CONTENTS HIDDEN) maps - and from there you can get very frequent and cheap trains to Amsterdam, The Hague, Leiden, Haarlem, Rotterdam etc. They all take about half an hour, hence a great base for holdays. Or you can just sit out on Carien's balcony and watch the world go by! Kitchen is extremely well equipped including a dishwasher, and Carien was kind enough to leave us plenty of basics including dishwasher and washing machine tablets. You don't need to bring your own! The washing machine/dryer, by the way, is inside the flat and not, as some US accommodation, down in the basement in a communal area.
I would be very happy to stay here again and to recommend Carien's place to any friends visiting Utrecht.