I use a dimmable electronic ballast and do two plant grows exclusively with a 600 watt MH/HPS light fixture. During the first several weeks of veg I run it with the MH bulb at 50%. Once the plants get a bit of size too them (around week 4) I run it at 75%. I don't go to 100% until I switch to Flower and the HPS bulb.
It's really a perfect sized light. Is a full 600 overkill from the get go? Yes. But down the road you'll be very glad you have it.