If you are looking for any reviews on a specific tool, Amazon.com has reviews, as well as Harbor Freight.
Of the two, Amazon has been compensating for reviews. I haven't heard that of Harbor Freight.
Perhaps Sears has a review system, but their site is such a mess....all I was looking for was 14mm impact sockets, and the search was returning 100s of spurious results.
The Menard's site has little or nothing in terms of reviews.
Home Depot and Lowe's may have reviews, but again, a really messy search system.
So, believe it or not, "Horrible Freight" may be the best place to research a specific tool's user satisfaction.
Since you asked specifically about air compressors, I would ask you "why not craigslist?". Sounds like you are looking for a "boat anchor" size of air compressor. In that case, you might a good deal on an Ingersoll Rand unit.
Oh, you didn't mention it, but perhaps you need to have single-phase?
One thing I would say, is STAY AWAY from any old Craftsman / Sears unit. For all intents and purposes, Sears makes no attempt to supply parts for those units, contrary to what us old white guys may imagine or hope.