This particular health club offers those staying in the La Defense area a legitimate fitness alternative. If needing to be in La Defense, we strongly recommend staying at a hotel with an appealing gym (e.g., Renaissance La Defense), however, we know that travelers don't always have a say in their accommodation. Since the Hilton La Defense does not offer an on-site workout facility, Club Med is presently the only option for its athletic-minded guests (reduced admission fee of 20Euros applies).
The word "unpolished" came to mind while touring this underground health club. The Technogym and Life Fitness cardio/strength machines are decent quality, but most are dispersed in a somewhat disorganized fashion near the reception area and beyond. Spin classes are even held in an open space that could be considered part of the lobby. Very little natural light shines through the club's atrium windows above the circular pool (see photos; not for lap swimming), and the workout spaces lack character. Still, a lot of machines are in stock, including about 30 Life Fitness stack-loaded weight stations, so expect the basics (ample equipment in a boring enivornment) and you'll come away satisfied.
Getting here: If walking away from the Grand Arche (toward downtown Paris) look for Entry #6 (see photo) just beyond the Convention Center (on the same side) and take the escalators down. You'll see the club's atrium and pool in the center.