Capital One has tiered customer service- they're great to their customers with their higher-end products and not so great to their customers using their lower-end products. Thus people love 'em or hate 'em. I'll weigh in as someone who's a student and dipping my feet into the churning game:
I started at 15 or 16 as an authorized user on my cousin's Discover card. Because of their ties to China's local credit/debit network it was decided that would be most convenient to user when visiting family over there.
Around the age of 18 or 19 I applied for a Costco AmEx under my own name and was approved with a $2000 limit. I've not asked for an increase since then as rules were already starting to tighten and I didn't want to rock the boat given their phenomenal service.
At 21 I applied for a Citi AA Visa and got it. This was my introduction to churning. Since then I've applied for two or three cards every six months up until now- I'm waiting until next spring or summer to do more.