Hey jstermx,
I was asking myself the same question just a few weeks ago. I ended up going with Sedo's showcase. It hasn't been a whole month yet, but I have seen a fairly dramatic increase in traffic to my offer page.
I've gotten ~40 hits on my offer page at this point after about 2.5 weeks on the showcase, which doubles the amount of offer page looks I'd had previously. It's very obvious when the name hits the homepage (the domains rotate) because the hits climb dramatically. When it's not on the homepage, the difference is negligible. It seems to cycle back to the home page about once every two or three days judging from the way the hits jump, and I believe it stays there for an hour each time. So if you do the math you end up getting about 10-15 hours of exposure on the homepage for your money.
At the same time, I bought a category showcase ($10) for another domain, to compare how the traffic would do. That one is also up in traffic by about 13 hits as a means of comparison -- however that's also about double the amount of looks on the offer page I had previously gotten.
Hits are definitely up, but so far no offers have resulted from the extra exposure, and of course that's what we're all interested in. We'll see.
ripley.