I subscribed to Lumosity for about a year and played the games on a near daily basis. I did notice significant gains in critical thinking, and just general mental acuity. Studying became easier, and I noticed I became slightly more decisive. Uhhmm, do I think it's necessary? No, not really. I performed just fine in school long after I stopped. I did notice that I felt this heavy feeling in my head, like a mental exhaustion, after playing the games for 1-2 hours. It's like the same sensation you feel after you've studied for a long period of time. But if you have nowhere else to turn and you think you really need a mental boost, I'd recommend trying it. Just my 2 cents though.
All in all, I think the mental "boost" you gain is pretty mild, and you also have to take into consideration that playing the games on a daily basis is really time consuming.