This thought occurs to me every time I see an application using Eclipse RCP. Have I made a mistake by spending so much time building my application from scratch. So many other vendors are leveraging the power of RCPs to build their applications. Here's a few examples.

1. Oracle uses Eclipse RCP for their BPEL product
2. Active BPEL uses Eclipse RCP
3. FlexBuilder uses Eclipse ECP

and many others. This allows them to focus on the business logic rather than worrying about such things as CloseableTabbedPanes, Dockable panels syntax highlighting, context-sensitive content assist etc to name a few. Never mind the end result gives the same effect as looking at a brand new urban citybuilt with lots of minarets and spires just because it was faster to build. That is only a small price to pay for other benefits realized such as faster time to market, smaller development cost, tested GUI layer etc.
Am I bothered by this thought. Not really. First of all, when I started developing my app a few years ago, I had no idea that such a thing existed. AFAICT, the RCP movement began quite recently. Further, Iam proud of my achievement and people just have to look at my product to see that Iam right. I'd like to believe that I have achieved something big, built a masterpiece as opposed to riding on the work done by others. Mind you, that this is not another instance of 'not developed here' syndrome. Its just that due to a twist of fate, I have come to become the owner of a cool looking product that I have developed myself without relying on anyone else.