Cliff's: Yes, it will probably work if you stick to it and do everything sensibly, but there are better ways to go about it, that are probably cheaper, too.
You will get more aerobic fitness from a cross-trainer and/or stationary bike, but they can cost a lot of money, and any piece of equipment that can only really be used for one major form of fitness (in this case cardio) is not working in favour of training economy.
IMHO, fitness is best defined simply by strength and conditioning. This can be further broken down into the following performances: force production, endurance and speed. And there's a sliding continuum between each. For example, "power" is the measurment of fitness in which force meets speed. Cardiovascular training usually lies somewhere on the continuum between speed and endurance, but force production can also be a component of cardiovascular training.
Somewhat (but not entirely) external to all of these is range of motion, which you won't get much of from cardio machines, but (perhaps surprisingly) you will get plenty of from properly performed strength training.
As Habib said, adjustable dumbbells are a much better investment. An Olympic barbell and power rack would be, too, but that will take up more space and cost a lot more than just the dumbbells (probably about the same cost as the cross-trainer, depending on who you buy from).
Here are some great dumbbell exercises to learn...and the best thing is, weights can be used to develop force production, endurance and speed, by either doing the movement with more weight, for more repetitions, or faster.
- DB Goblet Squats
- DB Step Ups
- DB Swings
- DB Overhead Press
- One-Arm DB Row
- DB Bench Press
- DB Upright Row
Utilising all of these exercises equally will improve whole-body function. You'll be stronger, leaner, have improved balance and more explosiveness. If you organise the exercises into supersets or circuits, you'll also improve aerobic fitness. If you do most exercises in an 8-15 rep range, you'll also improve anaerobic fitness. While squatting and overhead pressing through a full range of motion, you'll maintain, if not improve, mobility in the hips, knees, ankles and shoulders. While doing every exercise with good technique (which includes good lifting posture) you'll decrease the risk of injuries both inside and outside of training, and you'll improve your general posture. All the while, the skeletal loading of the exercises will improve bone and joint health.
Meanwhile a cross-trainer and a bike will only get your heart rate and breathing up, and will provide very little -- if any -- of the other benefits listed above. Your call.
