When it comes to hitting the gym, momentum is a good thing. Mentally it gets you there, and it keeps you day in and day out. Just be sure that your momentum is mental and not physical when it comes to your actual workout.
Slow controlled movements moved by a muscle (or group of muscles) are what builds strength and benefits the body, not momentum. Swinging arms or legs (or whatever it is you're working) not only defeats the purpose of resistance training in the first place, but it also puts you at risk for injury. Hence, the name weight lifting, not weight swinging. If you're finding that you feel the need to swing a weight to complete a movement, you're probably lifting too much weight. If you're on the treadmill and your legs are moving fast, but you don't really feel like you're working, it's time to pump up the incline. The belt on the treadmill can help propel your feet along and while this momentum does wonders for your ego, it's not doing nearly as much for your health.