Don,
Square has an initial limit of $2,100 per day in charges for manual entry with no limit on card swipe charges. If you exceed the limit then they hold the "extra" for 30 days before releasing it. To increase your limit you need to provide them with your business license and other information. I had my daily limit raised to $5,000 within 24 hours of giving them the extra information.
As for usage, I don't recommend it with Android phones as you have to type in the information for each charge every time you do it. I run mine through an iPad with the Square Register. This option also has the ability to hook up a thermal printer if desired to print receipts. What's nice with the iPad is that I can make categories and have preentered items so that all I need to do is open the correct category then select the charge I want. Square automatically does the math with multiple charges and gives a line item breakdown.
For processing charges, you can either use the card reader on your phone/iPad or you can manually enter them. The rate for swiping is 2.75% while manual entry is 3.5% with a .15 cent per transaction charge. While these might seem like big charges, they are well within normal rates and best of all there are no minimums you have to meet nor do you have different rates for Discover or AMEX. The authorization code is located on the signature screen to the right under the line and is real small making it difficult to read if you're trying to record it for your records so be prepared. When doing manual entry you should write Phone or Internet or some other reference on the signature line instead of leaving it blank. This gives you something to fight with should there be a contesting of the charges. Of course, when the client is present have them sign with their finger or a stylus. Receipts can be sent via text message or email to the client (and with register printed).
What you need to be most concerned about through is keeping very good records of your charges. Square does not provide you the name of the person who was charged, breakdown of services, nor does it show their email address (this is why you don't have to do all the compliance crap with them you have to do with others). Rather, all you get is an email stating you took XXX dollars in payment from card ending in XXXX on this date. This has been an issue when we've had to reissue receipts (which can be done for up to 60 days) and had processed several identical charges on the same day.
Another issue to look at is Canadian credit cards. They may have updated the software for the Androids, but when I was using it that way you couldn't put in the alphanumeric zip codes manually. They did update the software for that with the Square Register so that is no longer an issue we have and they always did allow processing of Canadian cards when using the card reader.
All things considered and with experience dealing with multiple other merchant accounts I highly recommend the square provided you understand its limitations and what you need to do for your business to make it work well.