John, I've used SEP since it first came out and I pre-purchased the upgrade to SEP 2 as soon as the announcement came out. I haven't had time to work with 2 as much as I'd like, but I agree with Mike that there's not much SEP can do that you can't do with CS5 or Lightroom. The big difference is that SEP lets you do it instantly, without the hassle.
There's one thing SEP does better than anything else I've tried: it can introduce a correct-looking grain pattern. The difference is that SEP takes account of densities and distributes its grain pattern the way film does. The other grain generators just plunk down in an even, globally distributed grain pattern, which never looks quite right. The grain feature isn't a big deal, but if you're trying to emulate, say, Tri X, it's a help. I also like the zone display SEP uses to show the distribution of your brightnesses. With that feature and Nik's control points, which Mike mentioned, you can do a really good job of tone mapping in less time than it takes to tell about it.
Russ resumed well. SEP is also a time saver and it generates a layer.