As an aside, where are you finding Ubiquiti APs for under $50? Craigslist?
I'm wondering if there's still marketability in stackable form. Do they even have switches of compatible footprint? Although I always dreamed of the day I'd stack my router on a switch from the same line to create a beautiful networking statue on my desk, I always deferred to price and other features when making purchasing decisions.
I would rather have a $300 fully open source, hackable device than a $50 locked down box, which is what recent WRT models were. Belkin's price is off, but they inherently get "it", they aren't afraid of letting their devices go and this is awesome.
Belkin seems to be charging a 2 to 3x premium when compared to comparable dual-band 802.11ac routers. If this work results in wider 802.11ac support in OpenWRT et al., then this will be awesome. If not, then Belkin is selling a decent bit of kit, but you'd probably be better off hanging up a Ubiquiti AP and using a tiny PC for routing.
Ironically, Belkin has a decent line of routers that are already flashable. I bought a dual band one new through their eBay store a couple years ago for $35. Threw Tomato (firmware of choice since it was released) on it and have been happy ever since: very fast, never locks up and supports dnsmasq.
Also, did you ever stack the WRT54s? I got the impression from random forum posts that that would overheat the stack when under serious load.