Don't have a LED set up for my tank yet, but have been keeping tabs on the breakneck pace of LED lighting development for several years & recently began the switch to LED bulbs for home lighting. I don't consider it a craze, the technology is maturing quickly and choices as to form, function, colour etc. are increasing daily. This is the advantage I look at when considering LEDs, the sheer flexibility of choice that's becoming available.
The CFLs (compact fluorescents) the government & energy savers have been pushing for a number of years now will soon themselves be pushed out of the market by emerging LED technology. Initially I was happy to switch from traditional filament bulbs to CFLs around the home to save a few bucks on energy, but found many of them did not last as long as advertised. Furthemore, each one contains traces of mercury & has a mini ballast built into the base. Hazardous waste each time you need to replace one. Pull one apart & you'll be amazed at what's crammed into the base, hard to imagine that they are so cheap now, considering the manufacturing complexity compared to traditional filament bulbs. Fluorescent tube fixtures are a bit better in this regard in that the ballast is separate so you're not throwing it out just because a bulb/tube burns out. To be fair, LEDs require a driver or power supply as well if you wish to run them on your AC house current since they are a DC voltage device. They should last a lot longer though, no mercury and with a little creative thinking, it is possible to run them straight from a DC voltage source such as a bank of batteries charged by a solar array, wind generator etc. No voltage conversion required other than to ensure you're using the correct level of DC voltage & current to drive the LEDs.
Look around & you'll see many of the big players slowly switching to LED technology. Automobile lighting, street lighting, factory lighting and so on. Eventually this will bring the price down to a reasonable level for the average consumer. Starting to see more high power LED E27 Edison base screw in type bulbs appearing on store shelves already. If one of these burns out, no soldering, a simple replacement like any other bulb. Although, If you pull one of these apart, you'll find the LED(s) is(are) replaceable with some soldering skills, so you don't really have to turf the whole thing into the garbage. The 'driver' built into the base of one of these can always be reused, unlike the CFL offerings. A year ago it was tough to find anything like that. The price is still up there though, discouraging people from spending their hard earned $$s on a single bulb. This will change as competition heats up & the various levels of gov't get on board with incentives to buy LED lighting vs the CFL 'craze' that's been going on for a few years now.
I recently bagged a few cool white MR16 type 4 x 1 watt bulbs along with a pile of sockets for a pretty good price on eBay. MR16 form factor is traditionally used with halogen bulbs as accent lighting but is a good candidate for aquarium lighting. Designed to run on 12 Volt DC, easily replaceable two pin plug in type bulb. Passive aluminum heat sink built into the bulb, optics included or can easily be removed/replaced by unscrewing the bezel. You could easily mix & match a bunch of these to dial in the power & colour you need for your particular system. If you have a variable power supply, you can dim them as well. All you need is one or more 12 volt DC wall worts of sufficient power to run the number of bulbs you're using.
If I remember correctly I paid just over $30 for 5 of these bulbs & about $25 for 50 wire sockets, shipping was free from China. Wouldn't take a lot of DIY effort to pick up an aluminum U-channel from Princess Auto, mount the wire sockets, get a few wall worts & voila, I have a pretty decent LED fixture for my 4' tank for $500. Dimmable if I wish & bulbs easily replaced. They might not be the latest Cree LEDs, but still very bright & hard to beat for the price. Still waiting for better selection & further price drop, but in the meantime I've mounted 3 of these over my kitchen sink. 12 watts of power for some fairly bright lights to do the dishes under.