Browsing reviews of the Dauphine Saint Germain on Booking.com, most of the reviews are positive. The biggest complaint is the size of the room - too small for the price. (A few people complain about street noise.) But most people love the location, and having stayed nearby myself recently, I agree with them.
I stayed at two hotels in the area on my last trip, actually. One of them was the Hotel Europe Saint Severin, which isn't far from the Dauphine Saint Germain. The location as I said was great in my view. Yes, my room was small, but I was traveling solo. I found a great last-minute deal on it in May on the hotel's own website for 80 Euros/night. If you book a place that has free cancellation, you might keep your options open in the week leading up to arrival and check hotels like the Dauphine Saint Germain again and see if their rates have gone down - or maybe they have bigger rooms for cheaper, if they aren't solidly booked.