Spoiler alert: I ended up loving the new sleep sack.
When the package arrived, I admit I was pretty impressed with how soft the fabric was. It's cotton on the outside, but the inside is lined with superfine Merino wool. I can't imagine anything more cozy. (Merino Kids recommends that you use wool-safe detergent and not put this in the dryer, but I have followed neither of those instructions with no ill-effect.) It's available in two sizes: ages 0-2 and ages 2-4. The size we received was the baby version, appropriate for kids aged 0-2.
It was big on my five month-old, but there are extra snaps on the armholes that tighten it up for a smaller baby, and the neck opening is designed to scoop down low so there's no chance of it creeping up on to baby's face. There is one regular armhole and one the other closes with two tight snaps.
The sleep sack closes with one continuous zipper that runs from just under the left armpit down to the bottom right corner. You can open it up fully so it's easy to lay a sleeping baby in it and zip around them. I haven't tried this sleep sack overnight on Finn, my sleep sack Houdini child, but I have a feeling that the zipper would have been a lot harder for him to figure out than the type that runs straight up the front.
Speaking of my three year-old, I happen to know that this sleep sack would have worked at least until the advertised 2-year limit...I had him try it on for proof. He's 38" and just barely too long for it. He would have been totally comfy had I zipped him into it (well, other than the fact that he was protesting about being in the "baby sleeping bag" that he sees on his brother every day.) In the end I think I might have actually saved money going with this sleep sack rather than a few different sizes of other sleep sacks.
A very nice feature of the Merino Kids sleep sack is a reinforced flap opening designed to allow a car seat or stroller buckle to fit through. I've also noticed that it's useful for middle of the night diaper checks without having to unzip the whole thing.
The number one reason that this sleep sack has dethroned my old favorite though, is that it regulates temperature SO well. It has kept Cormac warm but not sweaty through this terrible winter we just had, and is proving to work well in warmer weather too. I'm quite sure we're going to be able to use it year-round. This suggestion for what to put under the sleep sack seems to be working out well for us:
80 F - Diaper only
78 F - Short sleeved onesie
71 F - Long sleeved onesie
64 F - Long sleeved onesie + pajamas
62 F - Long sleeved onesie + pajamas
So okay, expensive sleep sack, you win. I never thought I'd say I'd be willing to fork over so much cash for a sleep sack, but after trying it, I admit that I totally would. After all, you get what you pay for, and in this case, you get a pretty sweet sleeping bag. Sweet dreams, kids.