We recently bought the Ski Erg after having purchased their rower a few weeks prior. I think it is a great addition for several reasons. It is also low impact, which I like since my knees aren't what they used to be. It is a full body workout like the rower, but with an emphasis on the upper body, especially the anterior half and heavy on core strength. You can work the legs less or more based on how you use it. The rower obviously works the legs and posterior upper body, so the combination of the two makes for a more well-rounded cardio workout. It is a serious workout and humbled me quickly the first time I used it. Have been planning to get the bike also.
There are tons of great videos out there to help you decide if its worth the investment.