My experience with firstbase has been terrible. I've incorporated my business through them, and while they promise company formation within 2 to 7 business days for C-Corps... the company did actually incorporate within that time (checked on the Delaware website). However, it might be that they're so overloaded that on my user dashboard it still shows "pending" (It's been 20 business days and counting..). Support is very weak, only available through email, and takes ~3 business days to get a reply (not actual answers... "your certificate if incorporation might have been mailed to you, we do not have tracking".
I'm afraid I'll miss my 83b election window since the company has already been incorporated but I don't have any of the post incorporation docs to fill the information - they are also supposed to share this pre-filled form for me, but haven't.
Overall, I heard there are other solutions like Clerky or Stripe Atlas that have a better track record. I deeply regret having chosen FirstBase just because they look like another cool YC startup.
