Xbox fitness is marketed as a free to use (until January 2015) exercise app for the Xbox One that makes use of the new and upgraded Kinect. However, when Microsoft say 'free' they mean you will need to have a gold xbox live membership. Following the year long trial period you will have to pay a one off fee for the workouts you wish to use.
The app itself is clean looking and very easy to navigate and find what you are looking for. The front page has several tabs, with a similar design ethos to the Xbox One OS, showing your fitness profile, workout library, message centre and previous workouts. You can use a controller, voice commands or hand gestures to move through these menus with, in my opinion, the controller being the easiest. The first thing you'll do is set up your fitness profile which consists of a basic set of questions (age, weight, sex etc.) which the app uses to keep track of your calories burnt, workout progress and measurements against the xbox community.
Once you've got the hang of the navigation and set up your fitness profile it's time to pick a workout. When entering the workout library for the first time it can be overwhelming as the range is fairly substantial in both type of workout and workout length. You can choose between quick '10 minute solutions' that include core training, kickboxing and cardio or go for the full on 60+ minute 'intensity' workouts aimed at the more experienced user. Some of the workouts require extra equipment such as weights and floor matts but there are also plenty that do not. There are also complete workout programmes available from celebrity trainers that consist of 3 or more individual sessions designed to be alternated to give the user a good overall training programme. I have sampled several types and styles and there seems to be a large enough range to suit most people needs and fitness levels.
After making the choice (I went with the Mossa Fight Workouts) it's time to do some actual exercise. The basic set up is similar to a fitness DVD where there is an on screen instructor guiding you through each exercise and giving pointers as you go. The obvious addition is the kinect which picks up your body movements in the top right of the screen as you attempt the individual exercise drills and grades you depending on how well you do. It is also smart enough to tell you where you are going wrong if you are not hitting top scores. For example, when doing the fight workout the kinect was telling me to punch harder for higher scores or pick my leg up higher during kicking drills. The kinect also has the ability to read your heart rate without a monitor on your body which is a neat addition, but you will have to be standing still at the time so it only works if you pause the workout. If you've experienced the original kinect the most noticeable thing will be how more responsive and accurate the new kinect is. Every movement I made was picked up and relayed with no lag, although sometimes I got lost when on the floor but that may be my playing space. At the end of the workout you are graded out of five stars and your scores are compared to the Xbox community. Also, there is a neat little feature that compares you to any of your friends who have completed the same workout, which adds a bit of friendly competition. Another nice feature is that each drill has a timer which counts down and is a good aid when you are thinking of stopping because the exercise is getting hard. The intensity of each workout is obviously varying but I have found the exercises to be hard enough that there is room for improvement but not too hard that you want to give up immediately.
Overall, the experience is enjoyable (in the way exercise can be enjoyed!) and the app is a great way to keep good level of fitness. I have been using the app weekly since launch and have found it a good alternative to going to the gym/running but I suppose that depends on your level of fitness and how much you want to put in or get out of the app. I would recommend it as a complete programme for beginners/intermediate exercisers and as a good addition to extreme exercise buffs.
As this is TA a quick note on the achievements. There are a total of 21 achievement for a possible 1000 gamerscore. They are obviously all related to the workouts in one way or another and range from completing challenges and scoring points to getting 5 star ratings and beating scores. There are only a couple of misc achievements, one for throwing 1000 punches and the other for balancing on one leg for a whole minute. What becomes apparent about the achievement list is that this there is a lot of time and effort required if your are aiming for a complete game. The mount everest achievement in particular is a biggy - after nearly 3 months I am still only 70% complete!! The progress achievement definitely make this one to avoid if you want quick achievement boost but if you are looking for a long term exercise solution the addition of the achievements is a great novelty.
Edit: as mentioned in the comments the kinect losing your body when doing floor exercises seems to be a common problem/bug which obviously lowers your score for the particular drill. Also mentioned was kinect issues with some punching exercises but I have completed several fight workouts without this experience.