We went July 2011. Weather was great - no rain, sunny every day, mid twenties mostly. At night it was about 15 - 17 degrees. It was quite humid and we put our air-con on in the room at night.
First time in Cardiff and it was amazing. The city is so modern and the shops are great. Make sure you visit the St David's shopping centre in the city centre. Also the bay was the best part, amazing - all the restaurants have a view of the harbour and it's great seeing the sunset. Also you're not far from the beach. It's called "Barry Island" where Gavin and Stacey was filmed. We went here once and the taxi one way was £30 but the train goes every hour.
Recommend Cardiff to others