Here is a good head-to-head comparison between Dropbox and Ubuntu One. I cannot speak to everything in the article, but I can say that I have Dropbox working on two Linux machines and three Windows machines (2 XP, one Vista) and have always found the installation process extremely fast and painless. I have tried to get Ubuntu One working on my two Linux machines, but for my home machine, it always seems to trigger a problem that kills X, which is not good.
The thing that Ubuntu One does that Dropbox doesn't is sync contacts, but that's not an issue for me because I use Google to store and manage contacts anyway, and Zindus to access them in Thunderbird.
So I'm in agreement with the take-away: Use Dropbox, not Ubuntu One.
That being said, if I invite you to Dropbox and you join in, I get a bump in storage space. So, please, get Dropbox.