Are there any downsides to jailbreaking a wii? I'm wanting to do it to my friends Wii (so not mine) simply so that I can stream movies and anime to it, but I'm nervous because if it breaks I'd have to replace it.
Has anyone here A) Jailbreaked a Wii, and B) Had any bad experiences with it?
Well, as long as it is out of warranty, it's not a problem. As long as the Homebrew channel is uninstalled when you send it in, it wouldn't matter.
There is almost no chance you will brick your Wii installing homebrew. It's easy to do as well. Just go here, select one of the loaders for your system menu version (Bannerbomb or LetterBomb is easiest if you don't have the software for the other hacks), and then read the using a loader section to get started.
I also recommend downloading the Homebrew Browser to put on the card so you can easily install homebrew.
I haven't dabbled around with the technical side of it since I jailbroke mine, but if it still works, use the Smash Stack exploit. It allows the jailbreak via Super Smash Bros Brawl, which I'm confident is a game that we all have for our Wiis. The chance of bricking is minimal at best.
As Jazzy said, install the Homebrew Browser. It'll make your life so much easier. Additionally, if he doesn't give two craps, also install USB Loader GX. It's a pretty awesome piece of software which allows you to rip and load Wii games from a USB hard drive. I'm in the process of ripping my library to Dolphin-readable files.