This is a great idea. I would use it. In fact, I want to build it! I've been reading about sentiment analysis as part of my research, and I had this idea literally the other day: I was looking for laptop batteries and I did not have the patience to search through some N number of Amazon/eBay/other storefronts. The second issue was the fact reviews range from 1 star to 5 star on the same product BUT a crucial fact was that more recent reviews are more relevant (for instance, if everyone from the past 3 months is rating it 1 star, vs. only 5 star where all previous bad reviews were from 3+ years ago).

So how would you define sentiment? A single score on a range from "good" to "bad"? Multidimensional, like a range of emotions? A major problem I've found in sentiment analysis literature that some assumptions/models yield limited/poor results even if algorithms are good.

Have you started working on this? I'd be happy to discuss it or your ideas some more.