Well it isn't exactly new. Monks from Tibet create sand mandalas as a form of meditation -- and then destroy them when they are finished because destroying the mandalas represents the transience of life. (Look at the picture here -- the monks are using straws to blow out the sand because the mandala is so detailed.)
I started coloring mandalas at my therapy sessions because I had anxiety and my therapist had colored pencils and mandala drawings to color. I found it so helpful that we have continued to use the mandalas in therapy. The therapist also participates, coloring her own mandala.
Although my anxiety is now mostly gone because of a medication decrease, I decided to get my own mandala coloring books and some decent colored pencils. I got 36 fine Prismacolor pencils and 36 regular professional Prismacolor pencils. I also got some coloring books that are mandalas or designs or of a spiritual nature. The manufacturers of the coloring books are mostly Dover and MindWare. I like the MindWare books better. I have just about finished with one of the MindWare books, Celtic Mosaics. I don't know which one I will do next. But I am being disciplined about it. I am finishing one book before going on to the other.
By they way, there is a great website directing you to all sorts of grown-up coloring pages. It's called Best Coloring Pages and it's at http://best.coloring.ever.com. Thus, it's easy to find online coloring pages if you don't want to spend the $3.95 or $4.95 for a coloring book at Amazon.com. The only problem with the websites is that there are a limited number of coloring pages appropriate for meditation and relaxation. There are some mandalas, but not many. There are Mary Engelbreit pages, which some might find enjoyable (I colored some of these myself and liked doing them) but they may not be for everyone (especially guys, hehe). The Mary Engelbreit pages are at http://www.maryengelbreit.com/FreeFromME/ColoringPages.aspx/.
Although it's probably preferable to color mandalas than other shapes, I have found that some of the rectangular designs from the MindWare books are sufficiently similar to the mandala circles in structure to be of use. I think anything with a balanced design is relaxing. It may also be relaxing to color some of the more spiritual designs that include more intricate patterns. I have one book of wizards -- both male and female -- that fits this category. It is called Wondrous Wizards and is published by Dover.
Good luck with your coloring adventures! What's next? Paper dolls?