I've been using them for about 1 year. They've been great for my size business, excellent pricing structure, and the two times I needed support, their service was excellent via phone (if you prefer e-mail support, though, they are awful--they do NOT answer their e-mails!). It's also pretty easy to get accepted as a merchant.
The only real drawback is the restriction on billing anything other than monthly. They do allow domain names, as they consider them a one-time payment. But all you have to do for those who really want to pay yearly is ask for a check or paypal payment. By the way, this policy is not a random evil one just for laughs, but in fact it's actually MasterCard's and Visa's policy (in exchange, I assume, for a good rate). It helps cut down the impact of chargebacks.
I would definitely recommend them for any small-to-medium size company.