I recently replaced my Superdrive with an additional HDD. I didn't spend too much time over the decision, I just figured that I use my HDD a lot more than I used my DVD player and I noticed that I was buying my software online via download anyway. The final straw came when I was running into big time storage issues converting video, etc. So after watching videos on YouTube and on the various sites that sell the conversion kits and reading the various discussions on the issue, I felt confident in cracking open the back of my MBP. I decided on the Momentus XT for my main drive and I relegated the Hitachi 7200rpm that came with my MBP to the media bay.
And you know what? It was pretty darn easy to do and once the new drive was installed, my Mac picked it up right away and I've never been happier. A couple of things I want to point out and this is my experience..
1. SSD vs HDD. I personally did not see the need to spend $1000 on an SSD drive when I really needed storage. So that is your decision. Most people place the SSD in the main HDD location and use it for the OS and programs. The spare drive is used for storage, ie. iTunes, movie and video editing files, etc. I have seen some discussion that the SATA connection to the Superdrive is not as fast, but you need to check that out on your own.
2. Battery life. I don't see any real reduction in battery life. The Hitachi, when mounted, only spins up when I need it. So, I suppose that if I was running both drives then it would impact battery life, but the same would happen if I was running the DVD for a movie or copying files so its a non-sequitur.
3. I found a whole slew of companies selling an adaptor for the Superdrive slot, I chose this guy on ebay at
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=220742814526&ssPageName=STRK: MEWNX:IT . For $30 bucks including shipping it was a great deal. The item fit perfectly, the holes lined up and it was the cheapest I found that wasn't coming from South Korea!
4. Installation was very easy. If you have the dexterity to turn a screwdriver you can swap out the superdrive for the HDD. Just make sure you have the right sized screwdrivers and most important that THEY ARE MAGNETIZED! Also, make a couple of run throughs on watching the videos.
5. I use an external DVD if I need to install something on my computer. Think about it, all you need is a 4G thumbdrive and you have effectively replaced the storage capacity of a DVD. Also, what do you find you do more, slid a DVD into your computer or plug in an external HD that you bought to store stuff on?
6. Dont worry about voiding the warranty. Unless you spill coffee in the machine as you are trying to replace the drive, you can always pull out the drive and re-install your Superdrive before taking the computer for service.
Oh BTW, there are companies out there that will also sell you (or include it in the purchase of the Optibay adaptor) a kit that allows you to keep your Superdrive that you pull out that turns it into an external drive.
I say DO IT! In the end it's much better to have 1.5TB max storage possibility.