Trello is one of those apps that you can dismiss too fast because you think it doesn't do enough, but which to come back to because everything else is too complex. I've been using it extensively, and it has almost gone viral around me. As in, every time someone needs to manage a project, I hear "Hey, what was this simple project management software you told me you're using for everything you do?", and there comes a new Trello user.
I even remember using it for managing my own time when I was working for a large Japanese company that was using excel spreadsheets to manage projects, so I could have a clear interface and use a quick and dirty piece of JS I wrote to generate the sacrosanct Daily Reports I was asked to provide.
Sometimes you need a detailed vertical app - like a bug tracker, or an applicant tracking system or a CRM. But a lot of times, you don't. You just need something simple because your needs aren't that complex. And using something you are familiar with is easier than trying to learn something new.
Trello is the always going to be a horizontal app. And sometimes people will need to move to something else to solve their problems. If you are using Trello as an ATS and you start getting 50 applications a day, you are going to go nuts. You need to switch to Jobvite or Greenhopper. If you have a team of 100 devs trying to track your bugs, you want to use FogBugz or Jira or Pivotal.
Joel used a metaphor that I like: The verticals are like stones on a beach and Trello is the sand that fills in all the space between.
In the future I think you'll see the Trello product evolving to support those other verticals and adding value (i.e. quick overview) in the way Trello does best.
At least as of mid-2012, HubSpot was using Trello exclusively for project tracking. Even then, Trello did more than enough to meet their needs. If you think you need a more complicated tool, you might want to rethink your process and workflow. /$.02
This has been pretty much exactly my experience as well.
I tried it out and didn't get hooked. Then I went to use a bunch other other similar pieces of software, and eventually came back to kick the tires again. I was pretty much 100% hooked at that point. I've been using it for almost everything for two months now. We use it on my team, and I use it for almost all my personal stuff.
I think I've gotten about fifteen or twenty people I know to use it. They really dig it as well. I feel like it's one of those pieces of software that really "gets out of your way" so to speak