Adobe has made its Reader, AIR, and Flash Player apps available as well, so you can work with PDF files, AIR apps, and Flash media just as Windows and Mac users can. In addition to Symphony, IBM offers a Linux version of Lotus Notes.
All these apps work very much like they do in XP, so your users will need just an hour or two to adjust to accessing them on desktop Linux. Seriously.
Ubuntu has a handy utility to add and remove a broad selection of free Linux apps, from FTP clients to graphics editors, so you don't have to hunt for them. (But the Adobe and IBM apps aren't in it, so it's not complete.) These apps self-install, so you don't have to switch to the Terminal and use sudo privileges and other arcane commands to install them. Sure, IT techs can manage this, but not your users.
I'm disappointed that Cisco's VPN client, which my company uses, has the kind of install that gives desktop Linux a bad rep. You have to know basic Linux commands to navigate to the files in the Terminal, use sudo to get admin privileges, and follow the convoluted install script. As is common with these Terminal-installed apps, there's little documentation, and the Web is full of contradictory and inaccurate instructions on how to install them. Cisco dissuades end-users from getting information at its site, so even after I procured a copy of its VPN client software, I couldn't find reliable instructions for installing it, so I gave up after 40 minutes. I had similar problems installing Parallels Desktop's UI tools into Linux. VMware Fusion uses a Terminal script, but the program runs it for you when you first install Linux, so that's less of an issue. (Note that neither product supports cut and paste between Linux and the Mac, as they do with Windows.)
Let's face it: The app selection for desktop Linux -- especially those designed for regular folks -- is very thin. You won't find BI tools, database apps, media creation apps, and so on, as you would for Windows or the Mac. If you think the Mac has too few apps to be used in business, you'll downright dismiss desktop Linux.
There is the beta Wine app that runs many Windows apps, giving desktop Linux wider reach, as well as the commercial CrossOver version from CodeWeavers. But the list of supported Windows apps is not huge. Moreover, supported versions are often one or more iterations behind what's currently available. But Microsoft Office, Project, and Visio 2003 are all on the list, as are Internet Explorer 6 and Intuit QuickBooks. I tried to install three unsupported apps -- Adobe Acrobat Pro 9, Adobe Photoshop CS4, and H&R Block TaxCut 2008 -- but had poor results. Acrobat 9 managed to install, but the license confirmation dialog box would not close, so I could never use the software. CrossOver claimed to install Photoshop, but it did not. And it couldn't install TaxCut. So don't count on Wine or CrossOver for more than Microsoft Office and supported apps.