Overall, they have adequate functionality and are easy to use, but they're generally anemic compared to the corresponding PalmOS apps. An exception is the Expenses application, which seems more useful than the extremely limited PalmOS Expenses application.
In general, I found them considerably more useful after I used some of the tricks on my Agenda Tips page for turning unnecessary services and processes off. That speeds up the whole Agenda, and the PIM apps benefit noticeably.
I should mention that I didn't really buy my Agenda as a PIM - my PalmOS machine (a TRGpro) performs that function admirably, and there's very little more I could ask for in terms of organizing my life (except perhaps wireless WAN connectivity so I can check my boss' meeting schedule and schedule a meeting with him on my PDA when we're not in the same city). I bought the Agenda for the ease of porting and developing on it, for the smoothness of Linux-box-to-Linux-box connectivity, for the playground factor, and for ideological reasons, since the Agenda software - including the apps - is all open-source (mostly GPL). When I decided to buy one, I was willing to continue using my PalmOS PDA if it turned out the Agenda wasn't really adequate in that area. But I was hoping that the PIM functionality on the Agenda would be adequate to replace the Palm for most of what I do, so I wouldn't have to carry around two PDAs plus my cell phone. It turns out that it is adequate, although I wouldn't use much stronger language than that at this point. :-)
Like the PalmOS apps, the Agenda apps do not (out of the box) support linking between records (from one person in the addressbook to their spouse, for instance) or between applications (from the appointment describing a meeting in the Schedule app to the note with directions to the meeting, for instance).
Not all the pages are done yet; check back soon.