You are correct, <$10 transactions are free and anything above is a flat $.25. Like I said, it's not that popular and you won't run into a lot of people who use it, but when you do it's nice to have an account set up. I've had 3 clients so far opt for dwolla when I gave them the choice out of paypal, google checkout, bitcoin, or dwolla and each transaction has gone smoothly- you just give them your 9 digit (?) account number and they can send money to it. I haven't integrated it into a shopping cart or anything like that- I intend on looking at the API but haven't gotten around to it.
I'm sure you've already seen
this page, but it covers pretty much everything. I say it doesn't hurt to have it as an option for your clients, you never know when someone will want to use it. I've only used it for transactions from $50-300, so I can't tell you how good it is for the numbers you're dealing with.