I've bought a few. For me iThoughts was by far the best app. So many ways to sync your files (cloud, email, itunes etc). It works with around 10 different desktop apps so you can sync your files and finish your mind mapping on your computer. Lots of color options, options to add hyperlinks, pictures, etc. One of my favorite features is that it cleans up complicated maps for you by organizing it and evenly spacing everything (an option you can turn on and off in settings)
It works well with both simple mapping tasks and more complex ones--in other words, the app scales itself well.
The developer is active and offers frequent updates, really making the $9.99 price point a bargain for my productivity.