Hard to say what would be a good reason for you, but I had used vim for a couple of years and really liked it. I still like its conciseness.

I became interested in emacs because of the good support for Lisp w/ slime, etc. Plus, I thought I'd prefer extending my editor with Lisp vs. vim script - I was right. I hadn't written any vim scripts while using vim, but I've quickly written many little elisp functions and bound them to convenient keys.

The dired (directory editing) feature is great. I like the way it handles buffers & bookmarks better than vim. I actually use the calendar feature.

I like how the functions you right get integrated into the help facility automatically.

The "Learning GNU Emacs" book is great.

The architecture seems better than vim.

I wasn't thrilled with all the "chording", but I'm writing new functions and making the keybindings comfortable for me.

The help facility seems superior to me. Dynamic abbreviations, window/frame control.

I'm sure there's more, but I only switched about 10 days ago :)