Lived in Cardiff all of my life so far, so have lots of experience
It's nice enough, it certainly is the best city in Wales by a long way for a 'busy' student life, it's a lot smaller than most other major cities in the UK, but in a way it's nice as you don't have to travel far to travel, basically yeah it's fine but if you are coming from a big city like Birmingham, London, Manchester or Glasgow then it can seem a bit small and a lot more restricted on what is available. But if you go from a smaller town then it will be a great experience.
Only issue is Cardiff Met being not in the direct center like Cardiff Uni is, it's close enough but depending where you stay you might need to commute a little.
If you have any specific questions feel free to ask, I don't go to any of the uni's in Cardiff though, but have spent 20 years of my life living in teh City.