I've tried many LED equivalent GU10 (and indeed Compact Fluorescent) bulbs in the past, all of which have been described as retrofit. And while this may have been true to some extent in that they can be used interchangeably, the bulbs always extended out beyond the end of the lamp holder by up to 1cm, and this quite frankly, looked terrible. However, with these bulbs, Philips have mastered the size perfectly and they fit perfectly into the existing lamp holders. Therefore, for this reason alone, if I was considering replacing all my halogen bulbs, the Philips GU10 LEDs are the only replacements I would now consider. Having said that, the technology does not come cheap.
Each bulb will save 45 watts (per hour) of power, and it appears that the generally accepted yearly usage of a light bulb is around 1,000 hours. Therefore, the total saving over the course of a year is 45,000 watts or 45kW/h (i.e. units of power). My electricity costs around 16p a unit, therefore the total annual saving is approximately £7. Based on the current retail price of £25, the bulbs would have paid for themselves in just over three and a half years, but should still have a lifespan at that stage of 36.5 years. This would suggest a cost saving (assuming the bulb does indeed last 40,000 hours) of £255 per bulb (This doesn't even include the cost saving of replacing a halogen bulb up to 20 times over this time period). Hence replacing the bulbs would be worthwhile exercise, but the upfront cost would be massive.
The bulbs also come in various colours and beam angles. This particular version has a warm white colour which is equivalent to normal halogen bulbs. However, while it says it is equivalent to a 50 watt Halogen, it is noticably duller, therefore would never consider replacing them with these. Instead, I would much prefer the 6 watt, warm white colour in the MASTER range and would buy that version over this bulb.