We stayed at the Avia Montparnasse for 5 nights.
First, the location is really good. The Pasteur Metro station is just arounf the corner, let´s say a 20 seconds walk. There is absolutely no noise from the Metro though.
But then, the location is more or less all there is. No noise from the Metro, but this is probably mostly because the noise from the street , from time to time, is so loud it covers it. Make sure to get a room facing the back yard, then the noise will not at all be a problem.
There is no AC, and on the seventh floor (the extremely small elevator only goes up to a floor between fifth and sixth floor, meaning you have to walk three winding stairs to seventh) it gets extremely hot. We had 25 degrees outdoors, but probably 35 degrees or more indoors. Opening the window didn´t do much either.
In the hallways there is quite a bad, musty smell, but it isn´t overwhelming. Let´s put it like this, if I had that smell at my house I would call the insurance company and a builder right away. But the smell is only in the hallways, not in the rooms. The carpets are quite stained as well, but also this only applies to the hallways. To summarize this point, the hallways is a lot worse than the rooms.
The rooms then, as said before we had a room on the seventh floor, I would have preferred pretty much anything else (because of the heat). The room itself was really small, but I understand that this is standard in Paris. The carpet looked clean, and so did the linens. The bathroom was my main concern. There were two light fixtures, one spotligt and one ordinary mirror light. The spotlight was broken, and all rusty. Looked as it hadn´t worked in a long time. The mirror light had a broken glass, and a flickering light. There were stains on the wall and on the floor in bathroom. The shower was so small that I couldn´t shut the door while showering (I wear a size medium and is slightly below average height). The drain was quite clogged which made the water level rise in the shower, quite disgusting. In addition to this all of the shelves in the bathroom were loose and lacked som of its fastenings. The overall condition was poor. The 1.40 m broad bed was quite comfortable. We didn´t use the TV , but from just having flipped through the channels once I only got 3 channels to work, two french and RTL. We used the minibar mainly as a fridge. It wasn´t the place were you spontaniously fetl like opening a bottle of champagne.
To summarize, the room was better than the hallways and the rest of the premises. The bathroom was the real downpart.
If you are staying here, you should really lock all your valuables in the safety deposit box (or even better, take them with you). I asked one of the cleaning ladies to open another room up, since our friends were staying there and I were to borrow their computer. She didn´t understand my English but she opened the room up anyways, gave me all the time I needed in the room (she walked away and carried on cleaning another room) and then watched me leave with the computer and charger.
When reading all this you need to keep in mind that the rooms are really cheap. Me and my wife would never ever stay here again, but our friends seemed quite satisfied. I would say that this is around the same quality as Formula 1, with the only difference that the rooms have it own bathrooms. If you are fine with Formula 1, you will be fine with this. If you are not fine with Formula 1, you will defnitely not be fine with Avia.