Textastic is a text editor designed for iOS programmers that provides you with multiple features. The utility offers you support for more than 80 types of markup languages and source codes, comes with a simple, nicely-designed interface, and gives you access to plenty of editing settings for designing the iOS applications you need.
You can paste snippets of code from other Mac apps, attach comments, evaluate math expressions, merge lines, and much more. All of the aforementioned tasks can be completed by going to the main menu of the program and selecting the preferred editing option.
The program also gives you access to plenty of font and text editing settings (line wrapping, text encoding type, tab width, and many others). You can also select the preferred output settings for HTML, CSS, XML or XSL syntax from the Preferences menu of the program.
However, there's a popular Apple product on the Mac App Store called Xcode which comes without a price tag, offers you dozens of useful features, and is intuitive.
With that being said, I don't think it's worth buying Textastic when you can use the aforementioned Apple application without paying a buck.