SeaMonkey still uses the "kitchen sink" approach. It includes the Navigator browser, the ChatZilla IRC client, the Composer HTML editor, a mail and newsgroup client, and an address book component all bundled into one big application. To get all the functionality of SeaMonkey using the separate Mozilla apps, you'd have to install Firefox, Thunderbird, the ChatZilla extension for Firefox, and a separate HTML editor such as Nvu (which is built using the Composer codebase).
One complaints I've heard is that Firefox and Thunderbird don't play together nicely in Linux. Depending on the distro you use, it can be a chore to get mailto links to open in Thunderbird from Firefox, and to get URLs to open in Firefox from Thunderbird. Having all the apps bundled in one suite takes care of that problem.
I spent a few days using SeaMonkey as my primary application for browsing, email, and IRC. SeaMonkey seems to be on par with Firefox for speed and quality -- I didn't have any problems with sites not rendering properly or AJAXified sites not working properly. I was a bit concerned that Gmail wouldn't be available, or that stuff like digg spy might not work properly, but everything worked just fine. SeaMonkey Navigator was also stable while I was testing it on my Ubuntu system.
I ran into one weird glitch with SeaMonkey's email client. When composing a new mail message, I was unable to type any text for the body of the message -- the composer window simply wouldn't accept input in the body field. This happened only once and went away after I restarted SeaMonkey. I was running the Linux build on my Ubuntu AMD64 system.
I didn't really see any major difference between ChatZilla in SeaMonkey and the ChatZilla extension, except that SeaMonkey's version of ChatZilla is a bit behind the Firefox extension; SeaMonkey includes version 0.9.67, whereas the most recent update in Firefox is 0.9.75.
One feature that might be compelling for users is the Roaming Profile support in SeaMonkey. SeaMonkey, like the Mozilla suite, supports storing a profile on a "Roaming Access" server. You can choose to store a combination of bookmarks, address book, cookies, passwords, browsing history, and much more. That's convenient for users who switch between machines regularly.
Other than that one-time glitch, I didn't find any bugs or stability problems with SeaMonkey. This is marked improvement over my experiences with the Mozilla and Netscape suites years ago, when the suite's stability was less than optimal. Even though I like the integrated suite in theory, this proved to be a major liability when the entire suite would crash -- not only interrupting my browser session, but also taking with it any email messages I was composing.
What's missing in SeaMonkey
On the whole, the differences between the SeaMonkey components and standalone Mozilla apps are not that great, but you'll find that SeaMonkey is missing a number of features that might be hard to give up.
For instance, SeaMonkey is missing native RSS/Atom support, so you can't use live bookmarks in the browser, or subscribe to feeds in the mail/news client as you can in Thunderbird. The search bar is also missing, which is a feature that I've come to rely on. I'm used to being able to just hit
Ctrl-k to jump to the search bar and run a Google search, and I missed that right away.
SeaMonkey is also missing the spiffy new "clear private data" option in Firefox, which lets you clear the cache, cookies, and download history with a single hotkey combination.
Extension support is also rather primitive in SeaMonkey. Though you can use some extensions with SeaMonkey, it doesn't include an easy way to manage extensions the way Firefox does. I tried installing the Sage extension to manage RSS/Atom feeds, but even though it appeared to install successfully, I couldn't actually find the application in the menus or sidebar -- and there appears to be no easy way to uninstall or disable extensions.
SeaMonkey also features the old, cluttered Preference dialog, though I suspect that's not going to be a major problem for most users. How often do you muck about in Preferences, anyway?
Another missing feature, and one that I consider pretty crucial, is the automatic update feature included with Firefox and Thunderbird. Browser security is a major concern for users, and having an automatic notification and update for security fixes is essential. The extension update feature is also handy for getting the latest version of extensions, without doing anything more than checking for the update, accepting the update, and restarting Firefox or Thunderbird. It's certainly not that easy in SeaMonkey.
As for the rest of the suite, while Composer doesn't have a Mozilla equivalent anymore, there is Nvu. Nvu includes a number of features you won't find in Composer, such as a CSS editor and form generation tools, but it's not well-integrated with Firefox and other browsers. One feature that I really like in SeaMonkey composer is the ability to browse to a page, then open it for editing in Composer. One thing that Composer and Nvu lack, though, is the ability to use scp or sftp to upload files.
The advantage of open source
The integrated suite approach has a number of advantages, and a few disadvantages, but I'd move back to the full suite in a second if it had all the features I love in Firefox and Thunderbird. As it stands, though, SeaMonkey is just a little too retro.
What's really impressive about SeaMonkey at this point, though, is that it exists at all. I'm sure most people can think of applications that they'd prefer to have stuck with, rather than having to migrate to new versions. With proprietary software, that's not an option in the long run.
In this situation, however, a group of developers has been able to keep the original Mozilla suite alive for all the other users who would rather have a bundled suite of Internet apps than separate Firefox and Thunderbird because Mozilla is open source, and because the Mozilla team has been cooperative in providing the infrastructure to keep the project alive.
Even if SeaMonkey appeals to only a small fraction of users, it serves as a reminder of what open source makes possible. As long as a few developers are interested in maintaining the application, it'll be there for all who want to use it, and that's a good thing.