Anyone using 1 piece base layer

I used to use one for overnights in the White Mtns (NH) and the Adirondacks (NY). It worked well for me at the time, but honestly, a 2-piece arrangement is far more flexible.

I think your right Valerie that the two piece would be more versatile.

I find I need less insulation on the legs too. I like light stuff under my rain gear rather than pants plus rain gear, but I might wear a thicker version on top; for example, Cap1 bottoms with Cap3 top, etc.

#ERROR!

Not since it was sized small enough to be called a onsey

I find I need less insulation on the legs too. I like light stuff under my rain gear rather than pants plus rain gear, but I might wear a thicker version on top; for example, Cap1 bottoms with Cap3 top, etc. Absolutely this.

All good onfo, much appreciated. I am not going to bother getting one.

#ERROR!

it's nice because you get coverage across the top/bottom gap and add a layer of core insulation without the bulk at your waist.

My opinion is a bit contrary in that it depends on just how cold conditions are. Yes, i need less on my legs than on my torso, but if it's 15 degrees or colder, i will most likely use baselayer bottoms. If it is dipping into near and negative F* temps, definitely would be using baselayer bottom. So, i fail to see how having a 1 piece baselayer suit would really affect that, since i'll still be using more layers on my top than bottom regardless… If i was going to be experiencing consistent Arctic like temps, i would definitely use the Cap 4 1 piece baselayer suit. It would be a bit more thermally efficient per slightly less weight than a two piece baselayer ensemble.

Whats nice about a onesie, particularly one with thumbholes and a close hood, is it seals off your skin from direct snow and wind. I also layer more on my torso, but I tend to prefer light baselayers anyway. I haven't used it for backpacking yet, but I've enjoyed my IO/bio merino ninja suit the times I've used it backcountry and resort skiing. I'm sure their will be cold enough winter michigan days to warrant cap 4 bottoms, so I bet the patagonia onesie would be great. Hard to beat the wicking/breathability of that fabric, and the warmth would probably allow for a lighter weight softshell pant. If you've already got a well fitting cap 4 tops and bottoms (hem that stays tucked), then the onesie makes less sense.

My wife wears the Patagonia Cap 4 onesie. Its a little lighter than her combined separates. She likes it but complains the drop bottom side zips are too fiddly to get fully zipped and the non-zip open gap still lets air/snow in. I see Patagonia has rectified this with the Fall 2015 version with a fullzip drop bottom enclosure.

I wear union suits all winter long, including on day hikes and backpacking. I'm wearing one right now. This one: http://www.swainsinc.com/store/union_suite_large.html only in red. The trick is finding one that's mostly wool if you plan to be on the move. Don't knock it till you try it. They're supremely comfy if they fit right. I bought some from LL Bean for my wife and sister too. I've had probably 6 union suits in the last 15 years. Love them!