on a Netgear R6300 and it has been very fast, great with signal quality, and the QoS features are working as expected.
Both the R6250 and R6300 have a dual-core 800MHz CPU, so they have the power to handle a decent QoS requirement without bogging down potential throughput too much. I'm satisfied, and it wasn't that expensive. If your situation isn't too terribly complex (many dozens of users and extensive QoS rules) then it might be a good choice.
The R7000 is even faster and supports external antennas, so I second that suggestion, but it's also twice the price of the 6250/3000, which can be found on sale from $100-$125 brand new if you're a good comparison shopper and/or patient.