Over the years I have bought proxies from a number of different companies. The typical problem with proxy companies is poor support/customer service and overselling. For semi-dedicated proxies they are sharing proxies with a small number of people. They quickly realize that instead of sharing proxies with only 3 people, they can make twice as much money if they share them with 6 people.
Currently I buy from BuyProxies.org. I haven’t had any trouble and their customer service is top notch. With that said, I still check the performance to make sure it’s about what I’m expecting each month.