I tried the PDF-XChange PDF Viewer's OCR function using the free version which I downloaded in the last quarter of 2012 and it wasn't bad but in my experience at least for English only OCR it was not as good as even older Adobe Acrobat Pro or ABBYY FineReader Pro9 (dedicated ocr package).
1. quite a lot of characters ended up being interpreted as 'illegal' (gibberish - unicode?) characters under XP, SP3 - or at least, that's how they appeared when for instance copying the text layer from pdf to notepad to inspect results;
2. it doesn't seem to rotate landscape pages to try OCRing them; and
3. offers no means of reviewing cases of marginal recognition accuracy.
This was on the basis of a pdf compiled from good quality Grp4 b&w TIFFs preprocessed using ScanTailor.
A month or two ago I was informed that the Free OCR included with the Free PDF-XChange Viewer "will be surpassed in functionality with the release of a planned licensed OCR plugin for the the new PDF-XChange Editor later this year after the Editor is released in March". I haven't followed up to ascertain whether the OCR engine in the free version stayed the same or would be the same component as in the forecast superior PRO version, but I note for the freebie, Current Version is now 2.5.210 (Updated on 5 Mar 2013) at:http://www.tracker-software.com/
-- I have not tested the latest version to see if the shortcomings listed above still prevail.