Plenty of opinions in this thread... Here's mine.
I've had a few ATKs over the years and have never had any reliability issues with any of them.
I presently ride a '97 ATK 605 DSES and it still amazes me that a machine this big can handle so light and agile and that the suspension could absorb this much abuse. I've had a long list of big bore thumpers and none even come close to matching this beast.
I also have a '90 ATK 406 MX, of which I've had 2, and they are, bar none, my all time favorite dirt bikes! The 406 gives me simple maintenance, superb handling, awesome power curve, and great suspension and brakes. The 406 takes a different riding style than any other bike but once a rider has adjusted to these differences it is a serious weapon in the woods or on the MX track.
My son has a Cannondale 440 that he likes.