I have Square and love it, though I also do have a "real" merchant services processing account. The trick with Square is to assure you do NOT go over the weekly limit otherwise your funds are held up for 30 days. You can get them to change that but it takes awhile.
Two clients of mine just started using Stripe - super easy to use and highly customizable if you want it added to your website so people can pay from there. No contracts, no annual compliance fees, etc.
I'll also Echo what VAinParadise wrote about Intuit Payment Network - payments made by check are only assessed a flat $.50 fee and deposited directly into your bank account. I wrote a blog post about it some time ago.