Stayed there this past week. Overall good service, including attentive staff. Lounge (which is open until midnight, by the way) has free internet (vs paid in the business center). The lounge is very small but wasn't crowded. Choose a different Marriott if it's important to be by the Tube as it's a long walk to all of them.

I was told I had a room upgrade, but the room I had was very small (not unusual for London). Also even though breakfast is available in the lounge, it's still free in the restaurant, where you have a much better selection.