I found it to be an easy & painless process. While the initial appointment may be months out, people cancel all the time and that opens slots so that you can often hop in if you can be flexible.

Under the presumption that you travel a significant amount, you will also find that most of the airport locations do handle walk-ins even though they may say that they don't. If you show up and there is a slot, they will take you, especially if everything is in order and they realize that you will breeze through.

Given that Pre-Check alone costs $85, the extra $15 for 5 years seems worth it even if the website is clunky.