I'm running the 825 kernel with smart reflex enabled and it's working fine. With the 850/450 one it crashed the first time I played a flash video through the browser. Disabled smart reflex made the 850 work ok. 900 wasn't even booting with smart reflex enabled (usually crashed just after the NOKIA logo), but I haven't tried with smart reflex disabled.
In conclusion, 825 with the battery life benefits of smart reflex was a good choice for me.