In case anyone is wondering, here's a picture of the cheap optibay I bought that DOES NOT WORK in my Macbook Pro 13" 2010:
http://dl.dropbox.com/u/3017621/optibay.JPG
I'm still not sure if it doesn't work because it's faulty or it's a design problem. I would guess it's a design problem, because it does boot and you can see both drives, but there must be some kind of SATA conflict going on, as performance is terrible and the computer would lock up every 10 or 20 seconds (for 10 or 20 seconds each time).
I bought the OWC Data Doubler and I must say it is absolutely worth the money. Since OWC is mac-oriented you know it's been product tested on the macbook pro (and their customers demand that it works). Plus they have a good return policy. And you know you are buying from a company that supports the Mac community. OWC is also great about shipping, I ordered the bracket at 5:00pm EST and they still shipped it out same day with $7 two day guaranteed fedex shipping.
The bracket itself is the sturdiest one out there, with screw holes in the right place for the Macbooks, so the drive itself screws into the bracket (2 screws) and the bracket screws into the macbook (3 screws). Very sturdy, and it even comes with the tools you need (4 screwdrivers and a prybar) as well as some extra screws for both mounting the drive and for mounting the bracket (which is great as I lost some screws in the process). Also included is a great color instructional booklet showing the differences between installing it on a 2009 or 2010 MBP. And OWC has a great video on their website.
And finally it looks pretty neat too.