It's not necessarily bad for you as Tetris does give you mind a workout in geo-spacial relationships (or more commonly referred to as "How Stuff Fits Together"). Even the Simpsons made a Tetris-related joke on this!
Unless you doing a speed run (like a 3-minute sprint OR a 150-line Ultra), the 5-10 minutes you're playing any not be enough to really give you a work out... depending on how the speed gears up.
On classic, incremental increases (like NES, GBY or DS versions of Tetris), getting to Lv 9 or higher is good... Lv 15 or higher is great. Lv 20 or higher is excellent.
On gradual increases (like The New Tetris for N64), you'll likely need to play about 20-30 minutes on Marathon to get a good mental workout.
I know it sounds cliche, but I prefer to work on the "No Pain, No Gain" ideal. If I'm not grind the gears in my head while playing, I'm not playing hard enough to be beneficial. Nothing hurts as good as nice mental meltdown after a high level game of Tetris... but that's just me.
While it was unscientific, my math performance tended to reflect my performance in Tetris when I was in High School. When I did good in Tetris, my math scores were good. When I had bad runs in Tetris, my math scores were disappointing as well (still acceptable, but just not up to my personal average).
Tetris won't help you the more liberal classes (English, Social Studies, History, ect.), but it could give you a boost in more logic-based classes (Math, Science, Tech).
Hope this sheds some light on the subject!