Yes, the rooms are small and you will climb many flights of windy stairs to get to them.

Amenities are mixed- if you need a TV or telephone, you are out of luck but they have FREE wifi and you can use their laptop downstairs in the lobby.

But for under 100 euros in a central part of Paris (convenient to the RER, in a district with charm), we found it ideal. The staff are friendly and competent, especially for ...well, being Parisian.

We stayed at the Hotel Sejour for 4 nights in early June. Bathroom and room were very clean and the water pressure good though the shower stall was a bit small. The staff were extremely nice and the free internet a bonus. There was no air conditioning but because of the noise outside we couldn't leave the window open, so the room can be stuffy and hot. We asked for a room on the lowest level as I had cracked my patella(knee cap) just before leaving. The lowest room (2nd floor) is 41 narrow winding stairs from the ground floor and there were other rooms above with many more winding stairs. Large suitcases would be rather difficult to get up. Our room was extremely small, only about 1/2 metre between the end of bed and the wall. There was very little space either side of the bed as well. Again, if you had large suitcases it could be a problem. I thought the location was good but is not actually in the Marais. I would probably look for somewhere else if I went back to Paris.

We stayed at the Hotel Sejour for three nights in early April.
Pros: excellent location in the Marais, very close to many sights and a great little area to wonder around. There's a reasonably priced little cafe across the way (Saveur du Pain) that was great for a quick breakfast in the mornings.
Very, very clean rooms, reception and bathroom. Tastefully decorated. Comfortable bed. Hot water, good water pressure etc.
Helpful and friendly staff who spoke English.
Free WIFI and a computer in the reception as well (note: the WIFI is very slow most of the time but fast in the early mornings)

CONS: we only had two (relatively minor) complaints. First, there was a pretty big step between the bedroom and the bathroom and there was no signage or warning which resulted in an unfortunate fall (onto a ceramic floor) and a bruised hip. It could have been much worse (ie. knock on the head) so we were fortunate. But be warned: look before you step!
Second, we had a computer with us and were careful to lock the door after we left the room. However, the cleaning person was not so careful and we came back to find the door unlocked. Nothing was stolen and we were on the fifth floor so it was unlikely that anything would have been stolen BUT it was annoying. Also, one day we came back to find two people in our room putting up a painting. Annoying again because they could have just waited until we checked out the next day. Or given us fair warning earlier.

All that being said, I would good back because for the price, you get a pretty good deal and that's hard to find in Paris!

One more thing: if you don't like stairs, be warned that there's no elevator but I think this is pretty common in Europe. We didn't mind the stairs to be honest. Good way to work off all the pastries....

I was pleasantly surprised with this hotel. It was clean and in a relatively good location to see Paris - a little bit of a walk to the Metro station. The room was not tiny - it suited our needs. There was a laptop in the lobby that the internet could be accessed through. The pillows were comfortable, the mattress was a little hard but not impossible to sleep on. I had heard the neighborhood was kind of rough but I never felt unsafe. I saw on google street view that there was a bar next door to the hotel - it worried me but we never had any noise issues (and I am very sensitive to noise) most of the time the bar looked closed by the time we got back to the hotel to sleep. Don't know if there is air conditioning since I was there in March - didn't see any AC units though.