The most important reason to maintain proper form during exercise is injury prevention.  In addition, proper form is the best way to get the most out of your time and your body.  When we align the body properly, we optimize its power and its strength.  Therefore, we work less and achieve more!

Few things to consider:

1. Think about what muscle or muscle groups are you targeting with each exercise?  And once you begin the exercise, do you feel those areas being workout?  If not, then adjust either your alignment or the weight you have chosen.
2. Don't compare yourself to the person next to you, compare yourself to you last week/month.  Have you achieved more?  Are you getting stronger? 
3. Using proper pace and control movement to ensure that your muscles produce force throughout the whole motion, avoid using momentum!  You will recruit more muscle fibers and get a greater benefit from each exercise.