I had one of the more modern, heavier duty Dutch bikes (a Batavus Entree) for a while in London, it was quite nice in a way (in that it was built like a tank and there weren't any others about). They're certainly good if you don't like maintenance (it had a Nexus Hub gear and roller brakes). On the downside they weigh an absolute ton, I mean not just heavy, they're super-heavyweight and really hard to manhandle, say if you need to cross a railway bridge. If you've ever ridden a Santander hire bike in London they feel and handle roughly similar.
If you mean the more traditional / retro Dutch bike (Batavus do one called the "Old Dutch") they're lighter, a little less bomb-proof, and even simpler, usually 3 speed.
I wouldn't want to regularly ride 20 miles on either one, or at least not unless I lived in Norfolk, and I was in "norfolking" hurry whatsoever.
They're likeable, but not designed for effficient distance covering.
To give a small insight into how most "Dutch" bikes are used I have a mate in Amsterdam who rides the bike below, but only for town use. He will do about 5-6 miles on it, maybe around the city to meet friends, bits of shopping (He says the front box is good for a slab of beer) and then ride back to his flat, but anything outside of the city and he rides a road bike.
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