I've been working on a full (!) roundup of all (!) PDF readers on the iPad.
So far, I've compared no less than 28 (!) PDF readers. For comparison, articles so far published (see e.g. http://appadvice.com/appnn/2010/04/ipad-pdf-viewer-showdown/ ) compared way fewer PDF readers.
As I plan to wait with publishing the final version of the article until Apple release the new, promised iBooks version with PDF support later this month, I've decided to publish the comparison & feature chart I've been working on (it's part of the article). It already has a LOT of useful information you'll find VERY handy. You may want to start with the bottom-most Verdict / Summary-row to quickly see which PDF reader to choose and which to avoid at any rate.
Please see http://winmobiletech.com/sekalaiset/201006iPadPDF/201006iPadPDFv1.html . Note that, unlike my previous charts (see e.g. my iPhone / iPod Touch Multiplayer Gaming bible's chart or the iPhone / iPod Touch Web Browsing bible's one), this doesn't let the user to dynamically hide rows / columns. This means that, currently, the chart may be a bit hard to navigate with all those numerous rows and 29 columns. The final version will already have dynamic configuration capabilities.
Feedback is welcome, particularly from PDF reader developers, who will want to thoroughly check out my complaints / bug reports so that they can easily see what functionalities I miss the most from their apps.
Once again: the chart is at http://winmobiletech.com/sekalaiset/201006iPadPDF/201006iPadPDFv1.html . Click the link!