I have been impressed by Lighten. One would expect the folks at Xmind know what they are doing, but honestly that doesn't always result in a decent mobile experience. Lighten is, as the name hints, a lightweight mind mapping app. It has simple feature set, but is a nice looking app with an intuitive workflow. For anyone looking to get into mind mapping on the iPad without fronting the expense for one of the more feature rich apps like MindNode or iThoughts, Lighten is a really good option. At the moment, with a back to school special still running, Lighten will set you back US$0.99. That is a serious bargain.
The one notable omission at this point is the inability to export to OPML, but there are still a range of export formats that should make that a moot point or all but the most steadfast workflows, and I would suggest that anyone with such a rigid mind mapping regime will probably be well past a gateway drug like Lighten One woudl expect the folks at Xmind. You can export to a Text file, Markdown, image, PDF, and of course both Xmind and Lighten file formats. Most mind mapping apps will parse the map successfully from one of those formats, and converting a mind map to an outline can be done with either Text or Markdown. I tested the transfer between Lighten and MindNode in a silly variety of ways and every time the mind map came out looking just like the cruft that made it from my mind to the so-called map.
If you are looking for a simple, aesthetically pleasing, and competent app to start mind mapping with, I suggest you take a look at Lighten. Xmind have not only done a good job with the app to date, but judging by the release notes the app is a going concern that will continue to receive love and attention. There might come a time that the 99 cents you put down here pays you back tenfold.