As a Cardiff lad, I'd say that CWL is a great place to base a low-cost carrier.
For low-cost the only option is Bristol at the moment and anyone who's ever made the journey from Cardiff to Bristol will know what a pig Bristol Airport is to get to from South Wales. The airport's out on the other side of the city down little country lanes and Bristol traffic isn't known for moving fast.
The catchment area will take in South-West and Mid Wales, so I'm sure there will be enough traffic to sustain it. If you've ever tried to squeeze yourself into one of KLM's Fokker 50's from CWL to AMS you'll know that there's no shortage of people who want tp travel through the Welsh Capital.
Let's hope that if the low-cost traffic is sustained and grows steadily they'll speed up plans to make the airport more accessible by road and rail.