Instead of asking if the iRex is "already obsolete", I think you should ask yourself "is it obsolete for me?" I'd cheerfully buy something that everyone thought was obsolete if it was the only thing that could get the job done.
You should probably figure out if annotating PDF's is really that important to you. If you were a businessperson, and you had a business need to annotate PDF's on a portable device, there would be no hesitation because your needs would be clear and there is only one device that satisfies them.
However, you are hesitating, which suggests that either your needs are not clear, or that none of the options satisfies your needs. If you're asking the forums to tell you what "the best device" is, I think the answer will be elusive. This is something people can only discover through hindsight, if at all.
Finding "the best" product also really means you have to know your priorities. When I first shopped around, I had a lengthy list of must-have features. Nothing existed that met all my criteria, so I had to lower my standards and then I bought the device that I feel is best for me.
FWIW, I decided to go the "stay cheap now, get fancier product when it comes to market." route.