We're a satisfied customer of CampaignMonitor. The only problem we can find is: When will it be possible to pay with American Express when using foreign currencies? Now only USD's are supported for AMEX.
Right now USD is the only option - AMEX requires different setup for every currency, and it isn't a simple process. What currency would you most use?
We use Euro's as currency. Isn't there some work around? And are there plans to use AMEX with other currencies?