MX518 ive had it for over 2 years and let me tell you its a wonderful mouse. this is my second one though..after 6 months i had to RMA mine to logitech because the laser died or something...
DONT LEAVE THIS MOUSE ON 24/7
also go easy on the right click...a lot of people including me had problems where it didnt break but it was like sticky...idk how to explain it..like it caught on something everytime you click it...it happened on my first one but my new one ive had for about 6 months and its perfect =P