I did the South Beach diet years ago before going on Metformin. I was fine - actually, I lost weight and got pregnant! Tried to do it again since going on Metformin. Oh my God! I thought I was dying! Headaches, low engergy - exactly what you are describing. I find that I need to eat enough carbs while on the Metformin, otherwise my bloodsugar gets too low. My doctor told me that there's no evidence that Metformin causes low blood sugar but I've talked to several other women who have it.
I'm staying on metformin for now because it has taken care of my two most troubling symptoms - irregular cycle and hair loss. But I have to eat some complex carbs now, there's no way around that for me.

I have been on Metformin for almost 3 years and I have tried a variety of diets.
The one that has worked best for weight loss and managing PCOS has been low carb/high protein similar to south beach. Right now I only eat an average of 90g carbs a day and over 100g protein. You have to play with it a bit and see what works specifically for you.
From what I have found here are a few of the most helpful things I have learned:

* By far the best thing is eating every few hours.
* Get rid of the starches (bread, potatoes, carrots, etc) Sweet potatoes are okay because they are not as starch and have more fiber, but I can can't eat them within 2 hours of taking metformin or they give me a headache. The rest will cause a headache no matter what unless I don't take my metformin.
* Eating protein with non-starch veggies (no carrots, potatoes) every 2- 2 1/2 hours helps with the blood sugar drops and minimizes the sweet cravings.
* Good low gylcemic carbs like barley and wild rice (1/2c /day in the morning) are great. There are a few studies out there suggesting that they help maintain correct hormone levels especially with PCOS.
* Most fruit, juice, high starch,sugar, or processed foods will cause a blood sugar drop about 45-60 minutes after i eat it so I try to avoid them.
* Learn to eat and think like a diabetic. Portion sizes, low glycemic, eating frequently. Keeping blood sugar stable is the goal. I will have a blood sugar drop if I wait more than 3 hours to eat. I even carry glucose tabs because of it - found they are easier to carry around. If I have to take something sugary getting protein/complex carb is critical or I end up with a secondary blood sugar drop about 45 minutes later.
* From what I have found higher protein diets work well because it doesn't cause insulin spikes and drops. But the right carbs (about.com has a section on south beach and it is a good list) are important too to help maintain a steady blood sugar as well.

Also avoid alcohol when taking metformin. To this day it will still make me sicker than a dog (even if I only have a little bit like 1- 2 oz wine) and it intensifies the effect of the alcohol and the metformin. I find I have to plan it and skip a dose if I want to drink. If I don't I will have a headache that will last 2-3 days.

Hope that helps.

On metformin I find I need to eat every few hours or I'll start to feel my blood sugars drop (I'm IR, not diabetic). This is what I do and it seems to help:

Breakfast - 9 am
Morning Snack - 11:30 am
Lunch- 1 pm
Afternoon Snack 4 pm
Dinner 7 pm
Evening Snack 9 pm

I don't get hungry on metformin, I go from fine to shaky, light headed & irritable. Hopefully if I loose some weight I won't need the met anymore.