I love my Flex. And it's ideal for me because I mainly want to track steps and sleep too.
You also don't even need to put it in to sleep mode anymore, it automatically picks up when you are asleep and when you wake up. I've made notes of the times I've fallen asleep (roughly) and woken up to check how correct it is and it's pretty accurate!
The band for mine has become damaged recently, so I need to get a new wristband for it. I did contemplate getting a new one completely, but this one is ideal for what I want. And the actual device works fine, even though I've had it around 18 months!
Although for what I do, I think if I changed it at any point, I would probably go for the Charge as it tracks floors climbed too, maybe the HR version actually.
But for the amount of activity I currenlty do, I don't need anything more than that