I've never tried Gnus for email because VM has always met my needs. It does an amazingly good job rendering HTML mail into something readable on a text terminal (the way I use VM and Emacs), complete with keyboard-navigable links that VM sends to an external web browser, and for the few times I need to see the actual graphics, a single keystroke sends the message's source to said browser.
Personality Crisis (which is nowadays included with VM, as opposed to being separate back when you used it) automatically rewrites the From: line based on the message I'm replying to. Using BBDB, Personality Crisis also does so when creating a new message based on the recipient. Mairix handles superfast searching of my decades of mail archives.
You know this of course, but I can't overstate to other people how nice being able to modify/extend VM with Elisp is. I rewrote the link-navigation code to handle all link types, and wrote the code that the abovementioned single keystroke invokes.
VM isn't perfect. I know that I could do all of the above with Gnus, and quite possibly am missing out on other features that VM lacks. Overall, though, I really feel like I have a superpower for email handling with it.