Good high-power LED's will involve a much higher upfront cost, and yes they use power and generate heat. I'm working on a canopy with 60 Cree XR-E LED's on a 135g tank, and I paid about $5.50 each for them. I'll get a medium light level (I'll adding more LED's later). There are also power supply and heatsink costs. The LED's should have a life around 11 years at 12 hrs/day. Each one of mine will draw about 3.7W the way I'm using them. I'm replacing a 4X96W setup. This is a pretty substantial DIY project. Like Corykeeper said, don't think that this can be done well with cheap 5mm LED's - it will take hundreds and hundreds of them and reliability will be a constant problem. Also, vendor claims for many of the 5mm LED's are ridiculously (even crazily) high.
My lighting costs for my 135g will be about $500, and will probably creep up to $700 when I reach the lighting level that is my final target. If you want cheap DIY lighting, go to spiral CFL's or other solutions. If you want a reasonable DIY alternative to the $4000 Solaris unit (for a 72" unit) for less than 25% of the cost and can give up some of the (very nice) bells-and-whistles of the commercial solution - AND you have the time for a bigger DIY project - then this may be something for you.
Since I'm not sure what the rules are about posting places where you can buy the good LED's at good discounts, send me a PM if you're still interested.