Since this thread is now a few weeks old, my guess is that you've already made your purchase. I'm so pleased with my Airport Extreme that I'll give a reply anyway.
1. It's extremely reliable. I've been through about five or six routers that I can briefly think of, and most were stable except under load. Running torrents or video chatting would cause many to slow down or freeze up, requiring a reboot at the end of a session or sometimes even mid-way through. Many required reboots every month or so even if activity was light. Prior to the Airport Extreme, the only router I've used that remained stable even through rough activity and over time was the D-Link DGL-4300 (now discontinued). That was a beautiful router... and of course, being newer, the Airport Extreme (5th gen for me) has even more features. I've stress-tested mine with video chatting for long periods, and intentionally ran multiple torrents with increased peer limits. I have not yet been forced to reboot the router over stability issues.
2. It's always hard to tell how quickly a new technology will be adopted. 802.11g was seemingly very fast, while 802.11n was drawn out over problems with the draft process (which led many companies to charge ahead and release "draft n" products). 802.11ac looks to be on-track to have a quick rollout. However, quick or not, everything that's new is expensive. Can you afford it, do you really need it now, can you really make use of it now?
My personal opinion is that wireless N is fine. I'd like more speed, but don't absolutely need it. For that matter, it's only been within the past year that the majority of my devices supported 802.11n. I'm not willing to undertake the expense of replacing everything (or even the core components) with 802.11ac devices right when 802.11ac comes out. My plan is to wait. 802.11ac will filter into devices as I replace them, and when I have enough 802.11ac devices (or when my current Airport Extreme starts fizzling out), then I'll upgrade to an 802.11ac router. It'll take more time, but it's the cheaper route