Hope this helps. Plus you get a story
My XBox is in my living room. I had a longer ethernet cord that I stretched across the floor from my router in the office to my XBox whenever I wanted to connect to live.
This worked really well until the day my dog tripped up the cord. This has happened before and has always been a risk, but this was the first time that it resulted in the scratching of one of my games (Borderlands

)
Long story short my brother-in-law let me borrow his wireless adapter and I have not noticed any slowdown in online play or downloading. Plus now the XBox stays out of the way and doesn't risk being bumped.
Also I was able to return Borderlands and exchange it for a brand new copy, which was immediately installed to my hard drive, in case anyone is curious.
So I would say it is worth buying a wireless adapter. You shouldn't notice any slowdown.