I use Frontier mostly in two ways: for its website management capabilities and as a CGI for WebStar on our Hawaiian language website, Kualono. Kualono contained only a few dozen pages when UserLand first released the website management tools. I loved the outline-based structure of Frontier, and that environment helped Kualono to scale easily and logically. It also allowed me to easily implement Kualono's "dual-language" interface, as most pages are available in both Hawaiian and English. What made me happiest about Frontier as a website management system was that it automatically converted our 8-bit characters to the proper escape sequences. Though BBEdit has translation abilities, I found it a pain to convert all of the 8-bit text to escape sequences, back again to edit, and then reconvert again. Frontier maintains all of our text in our custom font system, and re-renders it whenever changes are made.
Since converting to Frontier, Kualono has grown from a few dozen pages to nearly 500 page: information on our language, online books, our Hawaiian language newspaper, homepages for other Hawaiian organizations, and much more.
Several months ago I developed a very short Frontier script that makes it easy for webmasters to spool RealAudio files with or without the assistance of a Real server. Amazing what a few lines of UserTalk can do for you.
A few weeks ago I began looking into Unicode, realizing that as Unicode support became stronger in the coming years people would no longer need our custom fonts. A few posts on the Frontier-Central and ScriptMeridian lists led to a way to render the entire Hawaiian language side of Kualono as Unicode, something that impressed even those involved in the Unicode Consortium. So, even though the Macintosh does not truly support Unicode, Frontier allows you to generate Unicode from a website maintained in the Frontier ODB. Its still in the experimental phase, but quite an accomplishment for a few hours work.
Frontier also does forms to email for us in our online store, provides random numbers for an online game (interacting with Tango), among other utility functions.
I don't consider myself a very advanced Frontier user, I've never had the time to deeply delve into UserTalk, I just have too many other duties to just sit with Frontier for a week and dig into the language. I tend to start digging when I need to get something done, and have not yet come across a situation where Frontier has let me down. I seriously doubt it ever will.