Not for long however. Surefire with their new lines are moving towards lights that are not user customizable, easily anyways. Their older 6p, z2, c2 lights are what are considered the modifiable Surefires. Also the E-Series is all pretty much interchangeable, but there isn't much point to swapping those parts around. The new lines of Surefires, like the 6PX/G2X and the Fury etc., are designed so the head isn't accessible anymore. That means when a new technology comes out next year you are stuck with the light you had. With the older ones you could just buy a new dropin and plop it in the light. Overready has a whole company built around this concept, with some pretty awesome products.
HDS lights are awesome too. Not so much their numbers, no crazy outputs or wild runtimes or anything, but there is just virtually no better made light that can do exactly what you want it to do. I have a bunch of Surefire's and my HDS Rotary is what is always in my pocket. Until my HDS Rotary hcri arrives that is.