This is a pretty broad question, but I'll relay some of my experiences:
Project Support
SlickEdit can open most Visual Studio 2008 .sln and .vcproj files. We still have problems with solution files that have virtual solution folders and nested project structures. But I've created several VS2008 C++ projects (and upgraded/migrated some VS2005 ones), and SlickEdit can open them just fine.
The build and rebuild commands work fine, as do any of the commands that call devenv directly. SlickEdit will read the .vcproj and .sln files to determine which of the myriad Visual Studios are to be used with the .sln or .vcproj file. And this is working fine with VS2008.
However, for C++ projects, the "Compile" command doesn't work. Since devenv doesn't provide a command line switch for compiling a single C++ file, we are trying to build the cl.exe commandline via an internal macro process called vstudiocompile. Doesn't seem this is working in VS2008 C++ projects.
Source Control Support
What type of source control system are you currently using? Are you using Team System? There's an MSSCCI provider available for Team System 2005, and I've used that quite a bit in SlickEdit. So I'm hoping that a connection to a Team Foundation Server 2008 via the MSSCCI provider should work just as well. But I haven't been able to get TFS2008 installed yet, so I haven't done any direct testing.
Language Support
If you're doing a lot of work with C++/CLI (or with C# 3.0 for that matter), you're not going to get the same Intellisense support you're used to in Visual Studio. (Intellisense is MSFTs term; 'round here we call it "tagging"). We've got some improvements to C++/CLI on the books for SlickEdit 2008.
But even for "plain" C++, SlickEdit will not detect your Visual Studio 2008 installation for the purposes of creating a compiler tag file (where we bring in all of the C++ headers). But it's easy enough to add one yourself. (Screenshot attached)
1) Go to Tools > Tag Files, and click the "Auto Tag" button on the bottom right.
2) In the Tag Files dialog, under the C++ section, click Browse
3) Click Add, and call the compiler Visual Studio 2008
4) Set up the "Header File" to be the same vscpp.h as for Visual Studio 2003 and/or Visual Studio 2005.
5) Set up the Include directories for Visual Studio 2008, as well as the Windows SDK that was installed along with it.
