Save energy and less heat, yes, but you also get about half the yield for the power you do use. Not saying that's a bad tradeoff necessarily, just the way it is, and you should probably know that before you go buy 20 CFLs or something. In other words, with a 400-watt high pressure sodium light, the most you could expect to achieve in terms of yield is around 12-14 ounces, and realistically you will probably get half of that. If you use 400 watts of CFLs for flowering, the most you can expect to yield is around 6-7 ounces, and probably more like half of that...for the same amount of electricity used.

A 400-watt HPS light with a good bulb cranks out 55,000 lumens. You'd need like 25+ 26ish-watt CFLs to equal that amount of light and even so, it still wouldn't be as good because of the lack of penetration with CFLs.

Not saying you shouldn't just go the CFL route. If all you want to grow is 2-3 ounces or something every couple of months, they will do the job. I'm just pointing out their inefficiency disadvantages, especially in regards to flowering. Whether that's an issue for you or not is your call.