Credit card companies often specify different ways of calculating their interest charges. Unfortunately there's no industry standard at the moment although I think it would be a good idea.
Credit card companies are always cooking up new deals to position their products above competitors, but be sure that a great looking cheap deal may not all it appears to be when you read deep into the terms and conditions. With someone like MBNA Ireland, when you [broken link removed], there are detailed and usable online help pages to assist you in understanding how you will be charged.
Generally these terms are fair, just don't expect to get something for nothing and if you're really unhappy you can always switch providers. You may not gain much from that though as most credit card providers are similar in the way they charge interest. The best option is to fully understand a deal before you sign up and pick one that suits your own circumstances - use it to save money if you can then go elsewhere once you've seen the benefits out.