I've got a house full of 50W MR16 down lights, most of which are in groups of 4 on a dimmer. I put together a cost break even analysis for all these lights, which took electricity cost, replacement LED light cost and average daily usage rates into account. The break even point was generally decades, and even with my most used lights (kitchen) came out to just over 10 years for a ROE. As such, I haven't changed any of them for now, as I expect that in a couple of years time the cost of LED lights will come down quite significantly and a sub 5 year ROE will be possible. I'm such a nerd!