The location of this flat alone is worth renting. It's super central to everything and significantly less busy during the day and night than other locations.
Pros:
Location - it is absolutely unbeatable.
View - Fantastic! You overlook a little square (more like a triangle, really) that's excellent for people watching. You can also see the top bits of the Sainte Chapelle and the Notre Dame.
Check in: Super easy. Samuel met me at the flat, helped me carry my stuff up the six floors of stairs, and then showed me where things were and how to use it. Very easy. Check out was as easy as leaving the key on the table and closing the door.
Spacious for one person, comfy for two. Three could make it work, but more would likely feel cramped. Unless two of the people are kids. Kids can fit pretty much anywhere.
Basic supplies: So many clean towels! And there it a bunch of extra TP so you don't have to run out and buy that. The kitchen is fully stocked with dishes, cups, mugs, pans, electric kettle, etc.
Cons (TO BE CLEAR, these absolutely would not keep me from renting here again. I would do so in a heartbeat. I just feel like these should be mentioned):
Bed: The bed is a double - decently sized that could easily fit two. However, one side of the mattress has an odd indentation where it sags. Not a deal breaker, but it was a surprise when I went to bed that first night.
Shower: The water pressure is great despite the small leak from the showerhead. Again, not a big deal. However, due to the telephone box size of the shower itself, I do wish the handle of the showerhead was shorter so that you could soap yourself outside of the shower stream. Or turn around without getting wet. I also did notice a little stagnant water smell coming from the shower drain but it subsided as soon as the shower ran.
Mosquitoes: I have no idea where they're coming from. I think they come from inside the building somehow because even with the windows closed, they still seemed to appear. There is a standing fan in the flat, though, so mosquitos bites at night can be mostly mitigated with the moving air.
Upstairs neighbor: whoever lives upstairs must not be used to people below because they like to play loud music on the weekend. It's not awful, and it's not as loud inside the flat as it is in the stairwell, but it can be a bit of a bother first thing in the morning and late at night. Additionally, make the bed in the morning because some dust can fall from the exposed beams (which are really pretty, btw) when they're walking above and it lands right on the bed.
Basic supplies: You will need to supply all your food. Which is normal and expected. But I'm also talking about basics like tea, coffee pods for the Nespresso, sugar, basic spices, first aid kit, etc. Again, not a deal breaker, but my first night I was hoping to make something and there wasn't any olive oil. I think whoever stays in the flat also leaves food items for the next person. For example, there was an opened bag of rice, some sugar, and some honey. I added the olive oil and mustard. So hopefully, with more renters, they'll soon have the basics stocked.
The main building door uses a keypad code. But I soon realized that that door wasn't actually locked, therefore making the security of a keypad moot. That may be why the flat door has such a beefy deadbolt. But in any event, if you are a single female traveler like I was, this may matter to you.
TL;DR: Would 100% stay here again and highly encourage you to book here. The flat has a few quirks, but honestly, what flat doesn't?