This was our base for a wonderful trip to Paris! We stayed seven nights, then another night at the end of our trip, and absolutely loved it. Montparnasse is a great neighborhood -- lots to see, but not too touristy, Also a nice walk to the Jardin du Luxembourg. The hotel is on a street with two different Metro lines (one at either end of the street), so it was easy to hop on the Metro and be anywhere in the city in 10-15 minutes.
As for the hotel itself, we were very impressed. The room was small, but that's the nature of a Paris hotel room. The bed was very comfortable and the bathroom was good-sized. Climate control worked well, and double-paned windows kept any street or courtyard noise out. We also didn't hear any noise through the walls -- a very quiet, comfortable room. Housekeeping did a good job of cleaning up and refreshing supplies every day. Also, the free Wi-fi worked well in the rooms, and there was a computer easily available for guest use in the lobby.
The staff was also fabulous at Lenox Montparnasse! They were unfailingly polite and extremely helpful. They happily gave directions and answered typical tourist questions. Also booked our shuttle to the airport for us, and even stored one of our suitcases for the 5 days we were in Provence (we returned to the hotel for one night at the end of our trip). We will definitely stay here the next time we're in Paris! Also, check out the bar Rosebud a couple of doors down -- best old-fashioneds around!