It's suitable for vim users who want better vim experience. For all other I'd suggest vanilla emacs or Prelude.
Spacemacs is, as the tagline says, FOCUSED ON EVIL.
As a vim user, I'm also used to my editor starting up instantly. Spacemacs takes so long to load that it's laughable. And since GNU emacs is single-threaded, the whole UI is completely locked while it processes the massive amount of configuration needed to make spacemacs what it is. The only thing "better" about that is that it gives you an opportunity to take a coffee break. I'm reminded of the "code's compiling" XKCD: boss says, "hey! quit horsing around!", worker says "spacemacs is loading!" And this is with 8GB of RAM, 8 processor cores clocked at 3.2 GHz, and filesystem on an SSD drive.
And then you run into problems with many packages from MELPA. If some third-party package introduces its own keybindings, then you still wind up with odd emacs-style keybindings here and there, rather than Vim-style ones. Furthermore, Vim's leader key is backslash, not space, regardless of how you like to configure it.
So, what's "better" about it? The fact that it's got a "pretty" powerline statusline by default? The fact that it uses some dark colorscheme by default? That's not enough for me to abandon vim.
Emacs is typically started once, and then you open files in the same instance. With vanilla emacs you can use `emacs --daemon` to start it as a server, and then use `emacsclient` to open files. That way it should open pretty much instantly. I'm not sure what Spacemacs calls those.
EDIT: Furthermore, I really don't see how a Vim user is supposed to consider that an improvement. I can't picture someone saying, "I used to just run my editor when I needed it, and it started instantaneously. Now, it takes so long to start that I just leave it running all the time. It's so much better! Powerline! YEAH!!"
If people like Spacemacs, that's fine, I don't care. Use what you want. But I'm really getting sick of hearing that it's "a better Vim", because it's really, really not. In fact, it's so much not-Vim that whenever I use emacs, I prefer vanilla emacs, even though I've been using Vim for over a decade.