The Timhotel Montparnasse is actually pretty decent by Paris standards - I've stayed in quite a few places in Paris and generally the standard is lower than most other cities.
The room and common areas are generally clean. Traditional but slightly modernised. You get a kettle in your room, which is uncommon in Paris.
This is my second stay in this hotel and the last stay was better. This time, the room (the same time as last time) wasn't quite as nice and then the air-con broke down. Got fobbed off for 2 days over the weekend by one receptionist, who claimed not to be able to speak English or understand my French (which is admittedly poor), that people were looking at it and it would be repaired immediately and they couldn't change my room. Eventually, on Monday a different receptionist switched me to a room with working air-con, but only a twin room was available (annoying, tiny beds - smaller than UK single beds). I was told it was any upgrade (???) despite it being exactly the same as the previous room but with 2 tiny beds. I was i this room until the following weekend.
The night staff were actually friendlier and more helpful than the day staff, I found.
I found it annoying to have to stand at the door as people brushed past on the street until the receptionist noticed you or you rang the bell for them to open the door.
Montparnasse is great, Gaîté is very vibrant and a bit louder than elsewhere in 14th. Great access to metro.