For Penryn, I know some reports are saying end of this year, but I'm really going to bet on Feb-Mar '08.
Is it worth it? Well, depends on what you're living with now and what you think Penryn will bring you. As I have said about Santa Rosa, there were key things I was waiting on before I bought (which I did today). Those things materialized and I am quite happy.
If you're just waiting for Penryn because it's the "next best thing", you're going to be disappointed when it gets here because it will likely only be some percentage better than what's available now, just like this is only some percentage better than what came before.
If you need it now, buy it. If you don't, don't. If you're not sure, don't.