I've been involved in design of an ASIC that used a customized Tensilica DSP, circa 2009. The custom instructions were the main reason we opted for that core. There are a ton of other customizations that you can make with Tensilica so you end up with a core that meets 100% of your needs with very little fluff -- almost as if you rolled your own.

I do wish ARM would get in on that action.