In recent memory I have used a TP-link WDR3600, a BTHH5a and 2 different x86 machines with OpenWRT. Back in the day I used it along with dd-wrt and tomato on an actual WRT54g.
If I was starting with no compatible hardware now I'd probably buy the Archer C7 and have a play with that (would watch ebay for a few weeks if need be to get the cheapest I could). If it didn't meet my needs (it probably would if all I wanted to do was SQM) I could use it as a wireless AP and look at beefier hardware.
For me the killer feature of openWRT is the SQM with cake QoS, it turns my terrible 25/1 connection from one only suitable for a single user into one that can cope well with several people streaming/gaming/downloading and it just works.