To transfer domain, get a paid plan
Their responsive designs look overall nice but they lack a good deal of creative freedom. Every aspect has a predefined structure.
You'll need to pay in order to have no Site123 ads, such as the banner ones at top of site.
My take on SITE123
SITE123 is an excellent solution for simple projects such as portfolios and business sites.
You can add SEO easily! There's even the SEO Advisor, which scans your site for what can be improved. I found this feature extremely beneficial.
Because of my slight coding background, I wish I could use this with site123..