The AT101W uses tactile, but non-clicky ALPS black switches. Its popularity, is mainly due to low price and its abundance on ebay. The keyboard itself is unremarkable, the key switch action, while mechanical, isn't anything to write home about. But if you can get one for cheap, who cares.
I have a love/hate relationship with my AT101w, sometimes I think it's great, more tactile than a Cherry brown... other times I think it feels worse than your average rubber dome.