Ok, I'm pretty new to rooting, and I did it because I just got an iPad but I don't want to get another data plan, but I've tried these two tethering apps, and I've found that OpenGarden works on myTouch 3G rooted with ginger yoshi 1.5 (32b), but Barnacle didn't work for me. To be fair, rooting is all hackery. There's a lot of variables involved and there's different stuff to do on each device, so just because Barnacle didn't work on this particular device doesn't mean it's bad software; from what I've seen, it's quite popular. It's also possible I just didn't configure it properly, but of course, I can't find very good documentation on any of these apps, so maybe that's not all my fault.
Anyway – OpenGarden. At first neither my iPad nor my fedora 16 linux laptop connect, but both could see the SSID. I changed the encryption mode from Auto to wpa_suplicant, and that seems to work. On the fedora box, I had to restart the wireless for whatever reason. Why wpa_suplicant? No idea. When I first tried OpenGarden, I got an error saying something about failed to activate wpa_supplicant, so I switched to that, but I still got that error. But I figured I'd give it a shot with the new setting anyway, and it worked.
Barnacle. I tried Barnacle first. It seemed to work fine when I first tried it at home, but I didn't really test it much, just hit google and called it good. Then I was here at work, and the wifi was down, so I fired it up. It seemed to be working at first, but for some reason, the linux box would reconnect as an unknown client (name says "[none]"). Anyway, it still seemed to work, but then after a bit, the phone would reboot and the networking got messed up, so I would have to reset it. I think the phone was only rebooting after I actually went and checked my text messages, but I don't really know what caused it. Like I said, maybe I didn't configure it right, but I don't know what anything in there does, and it should definitely not reboot the phone, so I tried OpenGarden.
Well – I hope this helps someone. If you have any more info please do post, but if you have questions, I probably won't be of much help because I'm not too familiar with rooting and android hackery.