NetworkManager is actually becoming pretty awesome. It's great to be able to e.g., quickly configure my laptop for "internet connection sharing" with just a few clicks.
For the foreseeable future, I'm sticking with ifupdown on my servers though. I'm not opposed to NM taking over there; the fact that it actually checks to see whether the configurations it enacts were successfully applies means that in theory it can be much more reliable than ifupdown, which just runs ip/dhclient, etc., without checking their exit statuses. It's just not as obvious how to configure it without a GUI (there's no man page for the per-connection configuration files in /etc, nor is there any obvious replacement for ifupdown's hook scripts).