I am DMing Keep on the Shadowfell right now for one player. He controls two characters and there is one NPC. The first four hour session was only his second time playing 4th edition and he took out all the way to the kobold layer and beat them, finding out more about Kalarel at the Keep in the process. For the second session, I completely cut out the encounters on the far right side of the 1st level map (I think 9-13 are the numbers). I also took out the encounter in the long hallway before Sir Keegan because up to that point it was too combat based. Actually, encounters 1-4 turned into essentially one big encounter.
So through two sessions, he has done the kobolds, goblins, and zombies. He also completed the hobgoblin/spider encounter. I believe the module can definitely be completed in a day if you take out some of the useless 'fluff' encounters. I don't know why some people don't like Keep on the Shadowfell. It actually fell perfectly in to my campaign, and the results will shape how the campaign world changes. I like the adventure. It is your typical low level hack and slash adventure. It is extremely easy to follow and DM. My player is loving it, especially the rping opportunities that Sir Keegan and Splug offered. Go for it, but edit it as you, the DM, need too so it fits with your style of play. You should do this with any module, IMO.