I pre-ordered the WRT1900AC and have had it for a few weeks now. It is a very nice router to begin with. However, I did have a couple problems with it. I had a very hard time getting my phone to connect via wireless AC. It seemed to keep trying to connect to the AC radio via 2.4GHz causing problems. I'm not sure which device is to blame as this marks the first time I connected my phone to an AC connection. Anyway, a few wipes of the router and phone later, everything works perfectly.
I was very bummed when I started reading about the lackluster "open source firmware compatibility." Since the WiFi chip is made by Marvell, Linksys, and in turn, the OpenWRT developers, are having a difficult time getting the source for the driver as Marvell is known not to release them. This is the main reason DD-WRT doesn't come to a lot of routers. If they aren't Broadcom, they aren't worth trying for them. However, Linksys claims to be working with Marvell on getting them to release the source for it, and if that happens, this thing will be perfect. There have been a few builds made using the binary blobs that Linksys have supplied, but they are having WiFi connection problems. See here.
The router itself is great, but it can really use some third party firmware. The stock firmware is just too limiting. I'm holding on to mine in hopes that things will eventually really start rocking for this thing.