Interior Lighting with LEDs

Yes. I have them at my office for the front lobby as effect lighting. I used standard GU10 240v holders with a LED light unit. It looks the same as small halogen low voltage unit, but isn't. No transformers needed and very very low running costs. You won't get much light out of them, but as I said, I use them as a coloured effect lighting on the glass frontage which works quite well. If you wanted to use them as ceiling lighting, you might need to go 50/50 with halogens to get near the light levels you need. I put mine in 5 years ago, so they are probably a good deal cheaper now. They cost me £8 per bulb at the time.

I installed a number in my last house. They are not very bright and this needs to be taken into account. Also they can be very direct i.e. they are like a spot light.

Basically as I understand it there are two kinds. One is an LED that will fit standard lighting circuits. So for instance you can get screw types that will go into exitisting screw type fitments.

There are also ones which require a transformer. These are the ones I went with as it allowed very small leds less than 2 cms across to be installed directly into the ceiling. They only use 1 watt each and give off no heat so were good for ceiling mounting.

However I had 8 in the kitchen, plus another 6 x 20 led bulbs for standard light fittings and the kitchen was still far too dark. I had to give up and get normal bulbs for anything approaching reasonable lighting conditions.

They are great for mood lighting, i.e. to give a hint of blue on a wall.

It depends on what you want to use them for, and Rasher and Gary cover both sides of that neatly.

The problem with LEDs is the dismal efficiency and the colour-rendering. Contrary to expectations (and the marketing), LEDs currently ony offer about twice the lumens-per-watt (input) that incandescents do; compact flourescents offer about 5-6x. So - very samll electrical draw, but dismal total light output results, which they only get away with because the output is very directional.

The other thing, colour rendering, is because white LEDs are basically blue LEDS with an added yellow phosphour to fake a continuous spectrum. It isn't anywhere near, and the effect can be deeply unsettling if you wish to work or relax under such light. I still reckon good LEDS are 5years away...

...and 5 years isn't so very far away. In the next few years we will see filament bulbs become outlawed and total replacement with low-energy options, which will boost the market and competition enormously. It is very likely also that low-voltage lights that run from a transformer will also be given the elbow, as all the transformer does is lose the excess energy in heat, so they are also massively expensive to run.
Transformerless LED's are the way to go, and any new lighting installations should bear this in mind in order to be future-proof. Certainly don't waste money on low-voltage halogens with transformers at this late stage.