tired of products that kinda work in most cases? I sure am. This is another shining example of getting what you pay for. Yes, it seems a bit expensive for a simple adapter, and I'm guessing it would cost less if only more people needed something like this. I've looked high and low and this is the only adapter I've encountered that just makes complete sense.
My local shop sells a rail kit for the same purpose, where it's two pieces and you have to screw them into the drive on either side, then mount the goofy looking thing in the 3.5" bay (horrible). I found a metal adapter plate at newegg (Startech) for about $15 that was one piece (much better looking and easy to use) and you used the bottom screws to attach the 2.5" drive, then side-mount screws for the 3.5" bay. This provided better sound damping and heat dissipation, but still only mounted one drive and also you couldn't use it where 3.5" bottom-mount was needed. I have these in my Tower PC and they work great. (it looks like they have updated this model with bottom-mount holes since I purchased a couple years ago)
Then I found this - I've always thought "the 2.5" drives are so small, it would be easy to fit two of them in the space of a single 3.5", why doesn't anyone make an adapter for this?!?!" Well, apparently someone at Startech has a brain. This device mounts in ANY 3.5" location, and does so with nylon or plastic mounts which inherently reduce vibration. The PC-board construction is rigid and durable (protects your drives, won't bend out of shape). Out of the box you just use the bottom-mount screws and attach a single 2.5" drive and go. Or, if you're like me and want to put a 2-drive raid array in a tiny workstation with only one 3.5" slot, before you mount the first drive, snap out the two riser plates and use them to stack the two drives with the side-mount holes, then use the bottom-mount holes to attach the bottom drive to the adapter plate. It's a few extra dollars, but I know if I have one on-hand it works for every application without hassle.
I have since located this product SILVERSTONE SDP08 3.5 to 2 X 2.5-Inch Bay Converter. It appears to have the bottom mounts and is definitely cheaper. I think it would help dissipate heat better as it is metal, but probably will not dampen vibrations and may actually resonate a little if your 3.5" drive mounts don't have damping.