Can I continue to use Visual Studio 2008 to develop for SharePoint 2010? Or do I need Visual Studio 2010?
I understand that VS2010 makes it easier to deploy and debug but other than that what are the benefits of moving to VS2010? Is VS2010 slower?
I'd say that developing in VS2010 is quicker, but that is because of the ease you can debug & deploy, hence you spend less time creating/running a script to go ahead and package your project into a solution which is then deployed. You also have good intellisense of CAML, which again saves time.
It's not that VS2010 runs faster or slower, it's that it lets you develop faster.
I highly recommend going to 2010. You'll get the new SPIs (SharePoint Items) and deploying is tons better. I haven't noticed it being slower. The WPF ui in VS2010 is much better on the eyes too. All of our devs upgraded, whether they are programming for SharePoint or not.
Visual Studio 2008 works just fine. Just make sure you get something like WSPBuilder.