I recently stayed at the Carlton when business took me to London during Wimbledon and all the standard business hotels were either full or ridiculously expensive. After reading all the terrible reviews I was fearing the worst, but it turned out fine and wanted to contribute a fair review.
Sure the location isn't fancy, but it's right across from King's Cross/St. Pancras for easy access to tube and rail (a very short tube ride to Soho, Covent Garden etc.) and I found a number of nice pubs a short walk past the station down Euston Rd.
My room (#8) was a few doors down from the main office on the ground floor. It was quite small and basic, but was also perfectly clean and covered all the basics - twin bed, desk/chair, TV, ceiling fan with light, reading light, bathroom with stand-up shower. Other than a very slight noise from nearby plumbing - less intrusive than noise I've had at much more expensive hotels - the room was quiet and I had no trouble sleeping.
The hotel staff were very friendly and helpful, free wireless internet is available in the main lounge and the included breakfast (eggs, toast, sausage, ham, beans, juice) was great. In fact, I had to leave a bit early to catch my flight and she brought out a breakfast for me 20 minutes early. I couldn't expect much more for the price in London.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC