We build networks for a living (and have our own in a couple of places) and make extensive use of the units in/at/near the core. Specific experiences:
As a rule we use the most cost effective option unless we have no choice with kit - this means the Mikrotik units feature very highly.
Using the RB1x00 series works very nicely but we've had some issues with high PPS (40+ Mbps of VoIP) often causing the units to reboot. In such cases (and for most smaller core networks) we make use of a pair of x86 boxes as core routers and we are genuinely very happy in such cases.
The single biggest issue today is the problems with BGP on Mikrotik - they sometimes keep announcing routes after a withdraw is sent - and the don't propagate default correctly (both reported and I am very hopeful on seeing these fixes in a routing-test soon
The primary suggestion I make to people is to not get caught up in just how feature rich the Mikrotik platform is - rather than doing everything on one device *because you can* rather dedicate more units to specific features. As example, don't use the same unit for terminating all your PPTP/PPPoE users and let it function as DNS+DHCP+core router etc
The units are reasonably priced so use more of them for specific features.
In regards the bigger units from Cisco - the ASR / 7600 series and similar often do really well in terms of line cards and module options which are simply not available on the Mikrotik platform. If you build a network smart and avoid anything not 'ethernet driven' these units can for the most part be avoided. Mikrotik interaction with Cisco on the MPLS side is reasonable and is another reason to pick ROS over most other options.
If one looks at the potential capabilities of the CCR I think that it will be a very strong contender for network providers not too caught up in the 'vendor' wars in regards to brand selection. I suspect I'd look at deploying them specifically in terms of their bandwidth management/queuing work loads.
A side note - the RB1x00 series also makes very nice 'switch' replacements for doing small scale q-in-q deployments since Cisco/Juniper/HP q-in-q capable units are a hell of a lot more expensive.
Hope this helps.