I haven't built anything non-trivial with web components but I do like where they're going with them in principle. The biggest problem I see with WC is that they require native browser support to really function properly and I'm not sure it's really in Apple's or Microsoft's best interest to support them. AFAIK Apple hasn't made any indication so far that they will support the WC spec.
I think the biggest problem with WCs is they are standardizing best practices from 2011/Angular early days. Things have moved on and mutable two-way state that binds to a stateful DOM is no longer state of the art.
Microsoft already supports some of the WC standard and Apple will as well since it's being put into WebKit. Polymer is a great polyfill though it does have some performance issues (0.5 is much improved at least).
