I used Ubuntu for about a month (it was the first Linux I used at home) but then switched to another distribution (Arch), which is generally lighter and faster, though it takes a bit more work to install and bring up to a nearly-complete level and doesn’t have many of the nice GUI tools that Ubuntu does. I mention this because I’m a bit technically minded, so my experience may not help you in deciding if the switch will be easy for your wife.
From what I’ve seen of other people, the main reason for leaving Ubuntu is the need for some Windows-only program, where a Linux equivalent is found lacking. Based on your post, I’m assuming that this is not a problem–you understand and accept the differences between GIMP and Photoshop, and aren’t anticipating problems with Open Office compatibility with Microsoft’s formats. The other big reason is hardware compatibility: wireless networking can sometimes be painful, as can ATI graphics cards, certain brands of printers, and a variety of other peripherals.
Is it worth switching? If you’re a bit technically minded like I am, you’ll probably enjoy the amount of flexibility that Linux will provide. Other advantages include the security and stability that Linux generally has over Windows. I find that I like the way things work in Linux a lot more than in Windows as I get used to this OS. You’re given a lot more control over the system, which of course allows you to make more and bigger mistakes, but you learn from that and gain new skills that will help you down the road when you encounter other problems. A good article to read, although one that is a bit hard-nosed and may act to scare you off of Linux a bit (by mentioning many of the common objections to it) is Linux is NOT Windows. Go over to the Ubuntu Forums and read some of the questions that are being asked by some new Ubuntu users. While you’re at it, look over some of the Tutorials and Tips to see what is possible with Ubuntu, and read about other peoples experiences in the Testimonials section.
Will your wife find the transition too hard? If she doesn’t do any system administration the change should be fairly minimal. The default GNOME environment (the program Ubuntu uses to manage the desktop) will look rather different than Windows. This is fairly superficial however, and you can change most of them–there are even guides out there for making Ubuntu look like Vista. As a normal user, she shouldn’t have to use the command line any unless she does technical things (writing and compiling programs, for example). As the administrator, you will have to get used to the command line, so be aware of this if you are not already familiar with it.
That said, the best thing you can do is to try it yourself. I recommend downloading a live cd and playing around with it before trying to install. If you do decide to install, dual booting for a while would probably be the best option, since you can always return to Windows if you find that Ubuntu doesn’t work for you. Here is a guide detailing how to install Ubuntu without losing Vista. My advise is to read it over a few times before you try anything, make sure that you understand what is happening, or ask questions (maybe in this thread) if you don’t.
I hope this all helps. If you have any specific questions, I’ll be glad to try to help you out.