I have a Blackberry 8820, so this will all apply to it where device-specific information is mentioned. This is largely a copy-and-paste from an anti-Blackberry rant I wrote on another forum, with a few modifications.

The email is the only thing that is even remotely good about it. And even then, I hate that it forces you to either copy the entire message into a reply or have nothing at all. Forget infix quoting. And I'm not really sure if it works as well when you don't have a Blackberry Enterprise Server pushing email to it, which you most likely wouldn't. (It might — I don't know, but I kinda doubt it. Why else would they have this server to push mail to you if just accessing an IMAP server worked fine?) All of your mail (and contacts and calendars) will be conglomerated into a single lump. AFAIK, there's no way to keep different accounts separated. (There is a GMail client. It seems to work pretty well.)

Some people say that you never have to reboot a Blackberry. Well, that's true. But only because it reboots itself all the time. Did it to me at least twice on Tuesday. But that's okay, because it only takes like 5 minutes (literally) for it to actually finish booting. Even better, sometimes it just shuts down and forgets to reboot, so that when you pull it out of your pocket, you get to start the 5 minute boot process right then. And every midlet I've ever downloaded that was not marked as specifically for the Blackberry causes the device to either lock up or reboot.

The UI is awful. The main UI is a grid of folders. Some of the default folders have decorations that fit with the UI. You can customize other folders, but none of the options fit the UI, and you can't even reuse the folders that already fit. In addition, you cannot put folders inside folders. Also, for a device that should clearly be icon-oriented, it seems oddly text-oriented. Sometimes it kinda feels like Netscape 0.9.

The available apps are almost universally poor, but at least they are few. The browser is putrid. It's slow and it seldom renders anything correctly. Opera exists for it, and while it seems to render a little better, it is far slower: unusably slow in most cases. There are some Google apps for it. Most of them are pretty good, and are definitely the best apps of any sort I've been able to find for the device. There are a few decent search applications, but they almost all end up using the default browser, so good luck with that last step of actually viewing the information you found.

The only apps I can even vaguely recommend are the Google apps, Boopsie (a universal search application that rewrites the results so that the browser can display them properly), and MidpSSH, a minimal secure shell client. A weather application would seem an obvious fit for the Blackberry, but the only one I've found is the one from The Weather Network (apparently the Canadian Weather Channel), and it's only just good enough to keep me from deleting it. (It's supposed to keep itself updated, but almost every time I go to use it, it spends at least 30 seconds downloading what can't possibly be more than a couple of kilobytes of data.)

The 8820 technically has WiFi, but it might as well not. If the device has any cellphone signal at all, regardless of strength or quality, it will prefer the cellphone internet connection over the WiFi connection. I frequently have to turn off the cellphone connection at hotspots so that I can do anything with the device at all. I have verified by multiple sources that there is no way to prefer the WiFi connection. The one advantage its WiFi has over many other portable devices is that it supports WPA-Enterprise.

That's all I can think of at the moment, but I'm sure there's much more. I have seldom disliked a device so much in my entire life.