I use 2checkout and have been for several years. I'm very pleased with the service as long as I haven't had to go to customer support for something (they are slow on responses via tickets - call if you run into a problem).
They are inexpensive to get set up - it was $49 the last time I checked. All major credit cards plus online checks.
Payments come in like clockwork - autodeposit into my bank accounts on Fridays.
They offer recurring billing, you can use their system for manual invoices and you can integrate their service with most shopping carts/affiliate programs.
They also are now integrated with a service that allows people to defer payments (online credit approval).
On the offside, they DO charge a higher fee than Paypal or having your own merchant account. However, it is nice to have an alternative other than Paypal if you don't want your own merchant account.
If you do decide to sign up, would appreciate if you pm'd for my affiliate link. (They pay a whopping $9, in other words, 3 decent cups of coffee or 1 1/2 domains

)
Regards,
Sharon Tucci