According to Judaism, just like the Torah was given in Ktav Ashurit, the world was created by God with these letters and therefore every single letter has unspeakable importance. It's no wonder that a large segment of the Kaballa deals exclusively with understanding the Ktav Ashurit letters - this is a mystical font. In fact, Halacha states even a non-holy text that was written in Ktav Ashurit letters should be treated like a holy text, since the font itself is holy. That's how important this Ktav is.
But when it comes to prayer books, it's hard to find Ktav Ashurit fonts. For some reason, the Siddurim we use today are not written in Ktav Ashurit font; rather, every publisher creates a new patented font when it comes to prayer books and Chumashim. The most popular is Artscroll’s, a modern and neat font that is used in everything the company publishes from Siddurim, Tehillim or Chumashim (see pic in the right). But a few communities have pushed for the usage of Siddurim that use the Ktav Ashurit font, most notably the Sefardic communities and some Hassidic (more specifically, Toldos Aharon and Biali) sects and it’s possible to find them with a little effort.
It’s much easier to find Sefardi editions of Ktav Ashurit siddurim, but I’m Ashkenazi and I bought the two editions I managed to find. The first is a modern and computerized version (if you are looking for one online, see here), only for the weekdays, and the second is a copy of a hand-written siddur (which some rabbis favor over the computer version) - see pictures below. Being a Sofer, I use these Siddurim rather than the popular Artscroll version since I prefer to stick to the millennium-old font rather than a modern day invention… but that’s just me.
Computerized Ktav Ashurit Siddur