I installed a set of headers when my manifold went south. That was at least ten years ago and I kick myself to this day every time I drive it. It takes way longer to warm up and drive decent and the coolant heaters for the bottom of the manifold are not a good solution. I haven't had any leakage problems but I don't like the sound and I'm all but certain I lost power and mileage. I had dual 2.25" exhaust run so I know it isn't being "choked". I have contemplated putting a "Y" in and running it single but always feel that if I'm going to that length I should put a manifold back on and let it work like Toyota designed it to.
But that's just my opinion.
I've got some Yotas, some bikes and a drag car...