It's the spot
Just spent my honeymoon in Paris in March. We researched TONS of hotels, as this was the first visit to Paris for both my wife and I.
We decided on the K+K for a number of reasons including price, location and look. The location is ABSOLUTELY PERFECT. It is 15 feet from the Rue Du Bac metro stop on the 12 line, a bus stops in front of the hotel, and it's right on St. Germain in the heart of the left bank. Close enough to walk to the Latin Quarter, The Louvre, Orsay, etc., but far enough for some peace and quiet at night.
The room was a great size in my opinion. We opted for an Executive Double, which had a King bed, a couch, nice closets, bathroom w/heated floor. We were on the top floor, so it was quiet. It was about the size of a standard king-bed room at an American Westin, Hyatt, etc.
The staff was great. They all spoke English, if that's a concern. All restaurant suggestions were right on. They even got us into Guy Savoy, on a Friday. We asked on Thursday.
The interior is modern, as are the rooms. They have a restaurant in the lobby, but we never ate there. It's Paris, you'll eat better for half the price at any brasserie or vendor. Keep in mind though that it's not super-luxurious. It's on pace with some of the boutique hotels in NYC. The Hudson, the W Tuscany, etc. If you're looking for a hotel that's lush and lavish, this isn't for you.
There's a great butcher shop, chocolate shop, wine shop and convenience store right around the block, if you want to stay in for a night.
It was an all-around great experience. I highly recommend.