I bought this watson bulk film loader on ebay some time ago but didn't have the time (or frankly the nerve) to try it. Around the time I got the loader, I also ordered a couple 100 ft. canisters of some expired efke kb--one of my favorite b/w films. Since I didn't want to risk ruining either of the efke films, I also bought a canister of expired fomapan 100. This was about 1/2 the cost of the efke and figured that I'd rather test that than risk ruining the efke.
Though the person from whom I bought the loader said that it was NIB and/or hadn't ever been used, my biggest concern was whether or not the mechanism was light-tight. My second biggest concern was that the mechanism would be light-tight but I'd mess it up. In the end, I unspooled a roll of 24 exposure 35mm I had around the house and ran that through the loader before I messed with the canister of fomapan.
To ensure that i don't accidentally pitch the film canisters that I'm to use for the bulk loading, I mark those with some leopard tape.
I'm not sure how this all works out in terms of cost. The side of the watson box boasts that one can "reload cassettes for less than a penny a frame." Like any (or most) new endeavors you put out so much up front, it takes awhile to see the financial benefits. I don't recall how much I paid for the loader (maybe 20 plus a small shipping fee?), but the reloadable cassettes (got these from freestyle, not dx-coded) were 16 bucks for 25. Just had to place a second order for these.
Then again, saving a few bucks wasn't necessarily the biggest motivator for trying this. I wanted to try bulk loading and I also wanted to buy up the expired efke! If it were possible to bulk load 120 (i.e., if they sold a machine similar to the watson), I'd definitely be down for that as a way to control costs!