Our go-to rain jacket is the Outdoor Research Helium II jacket (read our review of it) due to its small packable size, minuscule weight and breathability. It is a mainstay in our daypack, but there is one thing that I wished it had, right from when we bought the original Helium 1 jacket... pit zips. So when I heard that Outdoor Research was expanding the Helium jacket collection, I couldn't wait to give it a try.
The waterproof jacket itself is made from fully seam taped 100% nylon 30D ripstop fabric with Pertex Shield+, which itself is very breathable, but on humid days the ability to dump extra heat is invaluable. The extra long zipper under each arm adds a little more weight to the jacket (257 g) vs. the Helium II (180 g), and each zipper has two pulls allowing you to adjust venting accordingly. Here I am mentioning the extra weight as if it was a burden (although OR sure worked diligently to trim the Helium II to be featherlight), but when you pick up the Helium HD you will marvel at its lightness.
There have been times in blowing wind and rain that I wanted to tuck my hands out of the elements, but couldn't with the Helium II jacket as it has no hand pockets. Thankfully, this request has been answered on the Helium HD jacket and the pockets are quite deep so that no devices or maps risk falling out. Both hand pockets also have mesh innards to aid in the breathability of the jacket. The hood now has a wire brim to allow better adjustability, plus a drawcord to cinch the hood a little closer to keep out the elements. Even in colder weather, the jacket fits loose enough to accommodate layering underneath without restricting your range of motion from paddling to portaging. Even while wearing a bulky backpack, the jacket was tough enough to not feel like it was going to tear, while still packing up to a tiny size within its own zippered Napoleon pocket.
If you are looking for the ultimate in portability, I still recommend the Helium II jacket, but if you want a few more of the versatile features of larger jackets while still remaining packable, the Helium HD jacket is for you.