I understand why people would want to upgrade their lights to the CFLs. It is a more efficient way to light your bays. I think we should look even further when doing a light upgrade. Replacing the MH with a new T5 HO fixture makes more sense. 2- 4 foot T5 HO bulbs 54 watts each put out 30% more light while using only 3 more watts than a 105 watt CFL. The bulbs last twice as long, 20,000 hours vs. 10,000 hours, and a T5 bulb cost is $3 vs $20 for the CFL. The T5 also has a higher CRI index. Between the savings on energy and the savings on bulbs you are further ahead to replace with a T5 fixture compared to converting to a CFL. Plus if you check with your local electrical supplier they probably have a program that will offset most of the cost of the fixture upgrade. Check out EPACT program which is a govenment program that works in addition to the electrical savings program.