I have to say, since getting the iPhone, I use most of my "unlimited data" plan to do rather mundane things on the web - look for directions, check movie times, check email, etc. One of the things that I have found invaluable is the ability to quickly look up infomation on the internet, especially Wikipedia (note: reliability of Wikipedia information is not guaranteed, and use of wikipedia for professional purposes is probably not suggested). I would pop open Safari, go to wikipedia, and look up what I wanted to. It is great, because Safari is the "real internet" so Wikipedia looks great.
However, that wasn't enough for the people that came up with Wikipanion.
Wikipanion is a Wikipedia-interface app, so you don't need to actually browse the website. You can click on the app, enter your search term, and get the article you want.
Just like "real" wikipedia, within the article you can link to new articles, you have images to look at, and you can look at related articles. However, rather than being in the Safari browser, wikipedia is rendered specifically for the iPhone, meaning you don't need to zoom around in the webpage to make the text readable.
Wikipanion also lets you change the font size, and email an article to someone if you want to.
So basically, it is Wikipedia, but iPhone-ized.
This app is free, so if you find yourself going to wikipedia with any regularity, this may save you a couple of steps. One function I wish it had was to bookmark or have a "favorites" option - articles that you may access frequently. Graphically, it is very simple and straighforward - entirely text based. A graphical interface on the bottom allows you to look at related articles, contents of the article you are looking at, and changing font size. It might be sort of cool to have a random feature added (say, by shaking the iPhone) where a random article pops up. It also would be cool to have a little more of a graphical interface (at least at the beginning, when choosing a search term or something). But those are minor quibbles.
Pros: Easy wikipedia interface, saves some steps from going through safari.
Cons: Don't get "the real internet" experience (if that is important to you).
All in all, I find no reason not to have this app if you use it even infrequently. Price is not a barrier, so aside from the few minor points listed above, this app does everything it needs to do. For that reason, I give it four and a half out of five stars.