I did try SmartReflex on my N900 few days and it worked fine as long as my max frequency was 805MHz.

However, I did not notice any battery life improvement (actually it felt shorter...) over my own custom overclocking profile I made for my phone long time ago.

OK. here is some information that I have read on TI DSP's designs I'm currently working on my daytime job, and how the SmartReflex works on them. The purpose of SmartReflex is really to improve the manufacturer (=TI) manufacturing yields allowing them to custom program the core voltage profile for each chip during production testing. Each chip is different, where some require slightly higher voltage than others to operate reliably at given clock speeds. The old method is to set a fixed power envelope and core voltage, and bin all parts to meet that requirement. Obviously this leads to fair amount of scrapped parts. Now, with SmartReflex they can program the optimal core voltage table into the chip itself. Some may require a slightly higher voltage, but as long as total power consumption is not exceeding their target, they can release that part as "good part".

When the SmartReflex part is used in actual board the core voltage power supply starts with some predefined (high) core voltage. The chip will now boot up and load its SmartReflex power management block. After this point the chip does use the VID control lines to talk to the core voltage power supply dynamically adjusting the core voltage with optimal values just for that chip.

Now when I did my own custom overclocking profile I did basically the same thing: I do have optimal core voltages for each frequency steps, so the SmartReflex does not offer any benefit. (Actually SR can be worse since they normally add a bit more stability margin than I did...) Therefore I'm not at all surpriced that I did not gain anything by enabling SmartReflex.

To summarize:

If you are using stock kernel and/or stock voltage profiles, you can gain battery life by enabling SmartReflex. (But it is not quaranteed... if you are the unlucky one you may have got the "bad" chip that requires the high end of voltages that are already in use by the default profile.)

If you are using a custom made voltage profile with overclocking kernel most likely you do not gain anything. You may actually lose a bit depending on how aggressively you have built your voltage/frequency profile.