Nice domain! Some general feedback from me:

• Make the input type "email" instead of "text"
• Add links to the testimonial author's Twitter
• Looks like you are using quite a few different fonts, font weights, and font sizes; I would make these consistent
• Is this free? If so, then add to copy
• The vertical padding around the error box (e.g. "Email address is invalid") is too big
• The copy under the title doesn't use full stops, which for me makes the page look less legit
• I would have a "Built by @petecodes" somewhere, given you are well known on Twitter!