I've been running opptimiser UI 1.2.7 since Creamy posted it here and haven't had any issues at all.

With regards to the SmartReflex switches this is what I've found with my N9:
SR1 disabled
SR2 enabled
Custom Voltage 1.30V
Maximum stable frequency: 1140MHz


SR1 disabled
SR2 disabled
Custom Voltage 1.30V
Happily purrs along at a frequency of 1250MHz. No reboots.

Supposedly ramping up the MPU_CLOCK (VDD1 domain) should be uneffected by SR2, which controls VDD2 domain. Yet I can gain an extra 100MHz when I disable SR2, just as Dragoss mentioned.