In fact, the vast majority of products I review actually start off in the beta phase. Typically the way this works is that a product is sent to me ahead of public availability, in some alpha/beta form (either hardware or software, or both). I’ll test and provide feedback to the company. The goal here from my standpoint isn’t to be so much a beta tester in the traditional sense (though, that usually happens as well), but to provide feedback on the product about functionality and gaps before it becomes available and/or public. In other words, tell the company the things that I expect everyday consumers will tell them – based on the thousands of comments and millions of product review page views I get each year.
One item to point out though, is that during the beta product phase, I’m not here to air the companies dirty beta laundry. Bugs are an integral part of a beta development cycle, and I fully understand that (remember, my day job is working in IT). As long as those bugs remain in the beta phase, there’s no desire on my part to note them publicly in some itemized fashion. However, once the product is deemed final, and if those bugs are still present, they should and are considered ‘fair game’ for a review.
Finally, yes, I’m fine signing NDA’s for pre-release products and technologies. Once a product is released however, that NDA no longer applies (except for unannounced product aspects of course). Media embargoes (i.e. release after a given time/date) are also fine.