great product, but quite buggy

i was bough my used mac at december, 2014. it came with old mac os preinstalled (i cannot remember version), so i decided to upgrade it for recent yosemite. after upgrade process is finished, mac didn't want to boot. after turning computer on, i see only cursor on black screen. i've decided to make pram reset, which helped, and yosemite started to work on my mac. it's quite comfortable to work on mac, but many troubles waited for me. in first 30 days of my mac experience it was randomly rebooted near 20 times. without any error messages - it just reboots while i'm working - so i'm losing my opened files. various mac os x applications was crashed many times. but most annoying thing is this random reboots. i've contacted support - they told me go to apple store for warranty repair. here another problem arised - seller didn't send some 'cheque with serial number' for me. mac os told that i have warranty until 15.04.2015, but i can't make warranty repair without this cheque.Read full review