I've had my LS about 10 days, and have all three battery packs (1xAA, 2xAA, 123A). Comparing to my X5, the LS throws a longer, narrower beam, with a much more natural color. The X5 overdrives its LEDs to the point where the beam color is on the blue side. The most impressive thing I can say about the LS is that it's the first LED light that puts out a beam that doesn't look like a LED light--if that makes sense.
Joe, the problem with the tough on-off operation is limited to the 123A battery pack. Reason is that the 123As positive "button" is larger and flatter than the AA version. ARC has been recommending an interim solution of making the hole in the foam gasket larger. They will be sending out a redesigned gasket to all owners in the near future. My solution to the problem was to drop a small (10-32 ??) nut into the hole in the gasket. That plus a little o-ring lube solved the problem, and I can now operate the 123A config with one hand. I've tried out the AA packs, but I'm more interested in small size/high performance, so I will stick with the 123A pack.
I think the X5 is an incredible value for $40, and it continues to be a good value if it can use batteries out of a dead Surefire. This is supposed to be possible--I haven't tried it. The LS is superior, but costs a lot more for the performance increase. Aside from the LS, the X5 is the overall best LED light I've seen at any price (well, nearly any price).
It's a safe bet that you will soon see other manufacturers using this new type of LED (Luxeon Star from Lumileds), and that prices will come down, and no doubt multi-LS models, which should be BRIGHT. LumiLeds also has an improved LS design (Lambertian) on the way that will be even brighter.