I really like this jacket for an emergency-ish shell, when the forecast looks good enough, or you'll be going fast enough to skip a full blown rain jacket. That being said, the lack of front pockets make it only an in-case layer for me, as I like to have pockets to warm hands/hold snacks on real rainy days. It is pretty waterproof for how light it is, and holds up well over time.
A medium for me allowed room to layer a thin synthetic puffy jacket and t-shirt beneath.

It keeps you dry and gives you no reason to not have an emergency rain jacket!
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From my wife: I’ve packed this jacket a couple times without the chance to use it yet in rain. Super light and packable. While I miss having pockets, having some kind of storage is still nice. Good just in case jacket for both on the mountain and while traveling.

This jacket is a neat colour (bright turquoise), but felt a bit more plastic than I thought it would and was a bit more sweaty than I had anticipated.
However having said that, it was very useful when hiking Bryce (in -14C), Zion, Arches, Grand Canyon and Yosemite National parks as I kept it in my back pack (it’s extremely light), and used as a wind break when we stopped for a break in the cold wind while winter snow hiking. I knew if I was really cold I had it in my pack and would feel warmer straight away. Although not for this purpose I also used to sit on in the snow. I haven’t tried in rain. It runs pretty true to size (I am 128 lbs and 5’6” and with the M I can fit a few layers underneath and still move. It would be even better if it were longer so that while wearing you could sit on it.

This jacket packs down to about twice the size of my phone - tiny! It's super light and has a loop when packed into its pocket so is easy to clip onto a harness or pack. I've only used it in light rain and wind so far, and have it more as an emergency layer when I don't want to bring up a tougher rain jacket. While I feel like it would withstand tons of rain, it is a very thin material so just would be more susceptible to tear. Always the tradeoff with lightweight gear, but hasn't been an issue!

Fits well and true to size - a small fit me well and is my normal size.

used this jacket for 600 miles on the AT, where it rains frequently. never had an issue with water soaking through. i wore this a lot as a wind breaker too which worked great! it also makes a nice light jacket for hanging out in camp and avoiding mosquitoes. super light and packable. wouldnt go on a hike without this!

I have this jacket as my emergency rain jacket. It's lightweight and small making it very packable. I typically throw it into my mountain biking pack, but also toss it in when going hiking too.

It's perfect because of it's size and weight. I am a huge fan of color and always having color especially when it is raining to brighten any day, so there are always fun colors.

It keeps you dry and gives you no reason to not have an emergency rain jacket!

It started to rain on the trail last weekend and when I pulled this out of my pack, someone asked if it was my pack shell and stood dumbfounded when it unrolled into a jacket. It packs down TINY. It's ridiculously lightweight and keeps you dry dry dry. I like the elastic on the cuffs (my last rain jacket didn't have that). The trade-off for size and weight is lack of waist pockets and no armpit vents. It's a great all-around rain jacket; it performs excellently and looks good so it's perfect for on-trail and around town.

The Outdoor Research Helium II rain jacket is the best rain jacket I own! It my one of my go to layers I wear it casually around town and it doesn't look out of place despite having minimal pockets etc. It packs down perfectly for hiking and backpacking and takes up absolutely no room in my pack while still providing the rain protection you need when a storm rolls in. I got the night/ultraviolet and its so cute, I get so many compliments. Fits true to size.

Love this jacket. Perfect Summer windbreaker/random rain shower protection. Very light. No lining. Just a light Summer jacket. I bought a Large which will suffice. It's snug. I probably could have sized up for a looser fit but feel like this jacket runs very true to size. I purchased Ultra-Violet and love the color.
Now for the one con!
No pockets at the waist!
Seriously.
Only a chest pocket so...hmmm.
Something to just be aware of.
Not a deal breaker but it seems odd?
The one other thing to keep in mind very much so is it does not ventilate at the armpits.

Will be easy to pack. Takes up almost no room when folded down.

I bring this jacket everywhere in the summer! It is super lightweight and compacts pretty small so I transfer it amongst my different packs. I use it for hiking, biking, and around town. Despite its lightweight, it has kept me super warm and dry all summer!

I bought this jacket to take with me on my hike through Washington on the PCT. Let me start by saying it is incredibly light weight. I had people ask me if it was really going to be waterproof or not. And initially I didn't actually believe it would be myself. But then (in true Washington fashion) . It rained. A lot. I threw on my jacket and wore it for three solid days. And let me tell you it is light. Very comfortable.
My only wish for this jacket is for it to have armpit vents (or whatever the technical term is).

This is the perfect jacket for travel or throwing in your day pack when there is a chance of rain. I found the minimal features to be just right. The hood is an ample size, and has a draw cord which is a nice feature. It stuffs very easily into the internal pocket. There is also a bottom cinch if you are in some real wind!

Incredibly light, stows in own pocket and clips to backpack, great for unexpected summer downpours. Comfortably cut. Definitely sweat when exercising but the small size/light weight convenience is an adequate trade off.

I bought the Helium Jacket after my North Face jacket lost all of it's waterproofing and even re-proofing it didn't work. I live and hike mostly in Colorado, so we don't have rain super often and it's usually not for long periods of time, so this jacket works well, as it's super light and doesn't take up tons of space in my pack. (It folds into it's own packet nicely.) It doesn't have much in terms of pockets or extra features, but since I don't wear it often or for long periods of time, it does the trick. Overall, I'd buy it again.

LOVE this jacket. I needed a lightweight and breathable rain jacket to bring on long wet weather trail runs and have handy on climbs when the weather turns and this fits the bill! It is so light - easily fits in my hydration pack or clips to my harness. Bonus points for the bright green color.

The material feels really lightweight, hard to believe it's as waterproof as it is - but so far rain beads up nicely on it (unlike a couple other light rain jackets I tried). Fairly breathable - no pit zips, but can always leave the front open or leave the bungies loose.

I don't really miss the pit zips or hip pockets on this - it's not what I bought it for. I have another rain jacket - a bomber goretex shell with pockets, pit zips, etc- so for colder weather, longer trips, etc I've got that one - but it's bulkier (can't clip to harness for windy belay stations and takes up most of the hydration pack or must be bungy'd to the back). I tried on the Helium HD in store and didn't feel those features were worth the extra $50.

Size wise: 5'5", 130 lb, I'm a S by OR's size guide but purchased a medium because I've got more of climber build/muscular upper body and it drives me CRAZY when things are tight on my shoulders. I'm really happy with the fit of the medium and it's loose enough that can fit a lightweight down jacket or insulating layer under it in cooler months.