Today I received some 10w Xenon MR16 bulbs that I had ordered from SailboatStuff.com (
http://www.sailboatstuff.com/lt_G4_MR11_MR16.html). Here's a quick review.
I ordered these bulbs for a couple of reasons. First, they were the only 10W bulbs I could find in MR16. My battery pack will only support 25w or so, and I think I'd rather have two 10w lights than a single 20w. I also wanted to see how bright the Xenon is; some people claim that it is dramatically more efficient than halogen.
After doing some comparisons in my darkened basement, my subjective conclusion is that the Xenon is not remarkably brighter than the plain old halogen bulb it replaced. This was an apples-and-orange comparison, as I was comparing the 10w Xenon with a 20w halogen. A single 10w bulb was noticeably dimmer than the 20w halogen, and two bulbs were about the same.
In summary, they seem to be adequate, and I think 2-10w will be a better solution than a single 20w.