I agree with cheapo in that you need to decide what your fitness priorities are and gear your training towards achieving those goals. If you want to get stronger, increase your cardiovascular endurance, while also keeping your physique sleek, then simplefit will fit the bill. If you want to get strong and build muscle mass, you'll have to lift weights. I don't want to start an argument over which is better because both methods will produce different results.
Personally, I never felt as strong doing simplefit as I did when I lifted heavy. Conversely, my cardio was never as good as when I was doing simplefit (also, thanks to simplefit I'll never sit at a lat pulldown machine again).
In the end, I think you need to decide what your goals are, research ways to achieve those goals, and then stick with it for an appreciable amount of time. There's no reason you should have to stick to the same routine forever. Lots of people have used simplefit as a springboard into other routines.
If you want to go the weightlifting route, try http://stronglifts.com/
. They focus on the the core lifts like squat, deadlift and overhead press, and also include some bodyweight stuff like pushups and pullups. This is the routine I followed after simplefit. However, when I take some time off from that routine I always come back to simplefit.