It's fun game, worth try. As was said, it's not really a typical MMO, you get experiences and credits for each battle, you invest experiences and into your tank development and credits for buying new equipment and better tanks, also your tank crew gains experience and evolve - that's where the similarity with MMO ends. Rest of it mathmaking and battles - which are combination of team deathmatch and capture the (enemy) base modes.
Best way to find out what is this game about is watch these three tutorial videos:
Part 1,
Part 2,
Part 3If you want to try the game, then when you register it via GamersGate, you will get one premium tank for free.
http://worldoftanks.gamersgate.com/