Yes, my Darlex bag from Winterfetish more or less doesn't stretch (they call it ultra-tough stretch), though it feels not like much stretch. But I agree with everything else stated on their website, it is comfortable and inescapable.
That said, I don't have much experiences with non-fabric full-body restraints such as leather.

I had this sleep sack, and it was super versatile and easy to use. In fact I had several of them and wore them all out in my touring kit years ago--so lightweight.

For more intense, long-term, I recommend this one (I know it's a UK link, but y'all can figure it out.) It's thicker--more bondage-y experience--but breathable for the skin, and, best of all, machine washable after containing hours of that sweaty bondage monkey in there.

Really?!? My bags (I have had two) were incredibly stretchy, which is part of whatbm makes them inescapable.

Not meaning to invalidate your experience at all, its just so radically different from mine that I had to comment.

That's awesome... I have no clue about duration because that is totally controlled by the Domme and is usually part of days of service rather than a session in and of itself for me.

To expand on international's answer...

I don't feel that darlex (which is latex coated spandex) is actually any less stretchy at the same weave/volume of the material. That does not meam that all darlex or "speed latex" is created equal, just that every manufacturer has different processes.

Maybe we come from a different perspective.
I started with the Spandex, which is almost as stretchy as a pantyhose, easily stretching 100% or more. The Winterfetish Darlex Sleepsack on the other hand, I would rather compare to a thin neoprene dive suite, maybe stretching 10% if at all.
The Spandex was so stretchy that I could free my arms, and once they are free, you are easily out. No chance with the Darlex.