If you are refering to Square, the credit card processing app, it works great.
I have seen other companies have the same thing (intuit, and I think Pay Pal now), but I had signed up with Square prior to those coming out.
The set-up is easy. The processing is easy. Payment to your account is received he next business day. It emails a copy of receipt to both you and person paying, with their signature, if they signed.
You can either swipe a card, or manually enter numbers. Pretty simple.
I would recommend to anyone who wants ability to collect payments via credit card.