I got a year subscription out of curiosity, I won't be renewing.
Don't get me wrong it doesn't seem bad. After doing a lot of the games for a few weeks I felt noticeably mentally quicker; but I'm not sure it's worth the cost of admission. There are tons of better indie games out there that will accomplish the same thing with much more entertainment value. The only advantage of Lumosity is variety, in that you can focus on different areas (visual-spacial, language, short-term/long-term memory, etc) at a one-stop shop.
For $75 (the cost of a year's subscription), I'd rather buy $75 of quality indie puzzle games and get the same effect with more entertainment value.