I use Foxit and I like that it loads much faster than Adobe and I really like the Firefox plugin.
AFAIR PDFX has it too. Last time I've installed it, I have been asked do I want it (not really;).
I was using Foxit for a while until I got annoyed enough with their default toolbar. I can't tell which versions but from time to time, with new updates, it is impossible for me to save viewer's layout and after each start I have to switch off all button bars. Not to mention that it's impossible to turn them off from IE*.
Another thing is that they know that Foxit has problems with searching for national characters (Polish). Once again: from time to time, depends on version.
It looked for me like they can't fix one bug and not to unleash old one once again. After that I've decided to use PDFX and... I am happy with it for two months now.
*) Yes, I need that because I am using IEView plugin within my Total Commander installation.