I'm wanting to get a hori hori knife. What details should I look for, what brands are good? I know I want a carbon steel blade.

I have one of the ones that was promoted here a year or so ago - https://permies.com/t/52154/Hori-Hori-Garden-Knife-Permies

It's stainless, not carbon steel.  But I absolutely love mine.  Really sturdy, exceedingly useful, and I keep it on a belt hanging by the door so I can grab it any time I go out to the garden.  

It's my go to tool for all small jobs or for carrying around with me 'just in case'.

That was a good one, Burra.  Full tang is what you want if you will do a lot of digging.

I was given this same one for Christmas year before last and I absolutely adore it. By far the most used tool in my garden. It still looks amazing, too, and is still sharp, after all this time and use !!

I give this hori hori 10 out of 10 acorns. It's taken me a while to write my review because I've got little ones at home, and I don't want to accidently leave a giant, sharp, serrated shovel lying around...and I tend to forgetfully put down things that I'm using, and I didn't want to accidently leave this some where they could get it.

But, now that I've had a few chances to use it, I can say without a doubt that I love it. I used it to dig in the half-frozen ground, and it did great. It's very sharp and comes with it's own sharpening stone to keep both the smooth and serrated blades sharp. That serrated blade sure is sharp, too! It came in really handy when I was making a mallet and spoon for the roundwood woodworking PEP badge. It's a lot easier for me to handle than, say, a folding saw or even a pocket saw. It bites rather nicely into the wood. Due to being curved, it's not the best for cutting deeply, but I was using it to cut a centimeter or less to trace out areas on the spoon, and it worked fantastically for that.

I love that it is full-tang and that it shows how deep you're in the ground by inches. I can barely wait for planting season and being able to put this awesome thing to more use.

I wish I'd gotten more pictures while working with it, but here's it with the beginning of my mallet. I'll try to update this with more pictures as I take them!