I currently teach for WITS and have been with the organization for about 4 years. The certification is one of a kind in that you get both lecture and practical experience. The hands on experience is what you do not get in most certifications unfortunately, and if so it is usually only on a one day workshop or a week. WITS is over a period of 6-8 weeks and you get hands on each week.
WITS also has many valuable and the highest accreditations for education in fitness that are available.
Some gyms or fitness mangers have not yet heard of WITS mainly because they are typically managers and may not even have experience in the fitness industry OR they have only heard of NASM due to their club being affiliated with the organization.. I would encourage them to check it out if not. It is a great program.
They are a good certification to have as a baseline as are many others. I think the more important thing is to keep up with continuing education that will satisy the needs or wants of your clients. Even if you are not getting certified in an area of interest of your clients, at least educate yourself with material that pertain to your population. Clients do want to know that your certified but want results on what they are there for or learn something from you. I feel that it is our job to do research on areas of interest for our clients. They have their own jobs to worry about and research.