I can attest that the new editor is indeed much, much faster. One of the simplest tests you can do is load up a single page in IE with the old version and check the size of the page. Then retest with tne new editor and you'll see the page size significantly reduced. Telerik has done a pretty good job explaining all the ways they've worked to increase speed, such as loading only the necessary scripts, etc. Todd Anglin, although technically an employee of Telerik, has always provided honest and factual comments on his blog. Here's a post he made about some of the speed improvements:
A short answer to your question is yes, it's worth the upgrade. That said, the upgrade won't be completely seamless. You can expect a certain degree of work to get the new version running; just how much depends on how much "customization" was done with the old version.
Hope that helps,