The overall service is absolutely below standard. It would be even for a 3 stars.
Overall cleanliness is only sufficient.
Staff is rude (from the concierge to the cleaners) and un-helpful.
My room had stains on the wall, dust in the corners of the bathroom, the bathtub had water getting stuck in the drain and - above all - there was a very strong, unrecognizable smell: I had to keep the windows open overnight. Upon my complains at the receptions, very little was done to help on the above items, with no follow-ups from the staff.
The elevators is a miniature of a real elevator, and is often busy because of the cleaning staff. During the morning (but even in the afternoon) you can find big bags full of towels and sheets blocking the corridors. In one instance, I literally had walk above one to make my way.
Breakfast is an absolute and utter disaster: it happens in a small, gloomy -1 floor room with no sunlight, with the smallest choice of food I have ever experienced in a 4 star (croissants, some yogurt, bacon and eggs, a few more - that's basically all) and an acceptable surcharge for any hot drink except coffee: i.e. you pay already 18 euros for the basic breakfast and if you want a cappuccino you have to pay on top. Ridiculous.
Given the above level of service, the hotel is unjustifiably overpriced (800 euros for 3 nights).
I have difficulties in understanding the high ratings here on Booking.com
If you are looking for a good (very good) spot to visit and explore Paris and you have no interest in a good service/hotel experience, then I could still recommend this hotel. Otherwise, avoid it.
The position is simply perfect, walking distance from all the best of Saint Germain de Pres: bars, restaurant and even the Jardin de Luxembourg within a few hundred metres.
The decor is fine and overall the hotel would have the potential for being charming but...