I'm not using Spacemacs, but rather Evil mode in Emacs. I think the biggest thing I would caution is to expect small things to be different. For example underscores are word boundaries in Emacs. File selection is different. Some modes do not have vi key bindings set up by default. Escape doesn't always work to abort (you need to get used to pressing ctl-g). Omni complete is absent and completion in Emacs works quite differently. Searching through files and sorting through compilation errors works quite differently. I think that's most of the issues I've run into.
It took me about a month to get everything set up the way I wanted it and to get used to it. With Spacemacs, I imagine that things will generally be set up, but that it will take longer to get used to because it is pretty opinionated about things like leader keys. Right now my set up is good enough that if I accidentally run vim instead of emacs, it takes me about 15-20 minutes before I realise it (normally when I try to update an org mode file ;-) ).
My biggest advice is to take the transition slowly. Use Emacs/Spacemacs until you get annoyed with it and then put it away for a while. Eventually you will start to get used to Emacs-isms (like ctl-g) and Vim will start to annoy you. At that point cut over entirely.
One thing that's really annoying is that when you are modifying your .emacs/init.d file, it's easy to get it into a bad state. At that point Evil and also probably your theme won't be set up properly. So you have to fix everything using emacs keystrokes. I used emacs for 20 years before I switched to vim and even still it seems I am no longer able to use vanilla emacs ;-)
One you get it set up and are used to it, though, it works surprisingly well. For me the absolute biggest advantage to emacs is Org mode and I basically can't give it up now. Good luck on your switch!