I just switched from Cox to Century Link. Cox had recently increased the cost for my 50 megabit plan to 73.99 per month. I was able to get a 40 megabit plan for 36.75/month including the modem with my Direct TV subscription.
Switching was smooth and easy. My kids didn't notice it.
I used an internet network speed test to measure the speeds of both Cox and CentruyLink. With Cox, my peak download speed was 50 mb as advertised. With CenturyLink the peak download speed is consistently about 35 mb for downloads (compared to the advertised 40 mb). Ping times are longer with CenturyLink too (44 ms vs 11 ms for Cox).
Bottom line, CenturyLink is fast enough and the transition was painless.