Hey not to rehash an old topic but I thought I'd share my 2 cents.
Picked up the OEM Subwoofer for my '08 WRX, and installed it tonight. Took about an hour & 45 minutes including time to get tools together & clean up. The hardest part was getting the center console off without scratching any of it, but I think it was a success! The wiring harness makes installing it REALLY easy if you know how to use a screwdriver and a torque wrench.
For any nay-sayers, the sub DOES make a noticeable difference in the sound quality of the system. No, it's not a world of difference and it certainly does not sound like the kind of subs I used to have (ie. people will still see you before they hear you), but the sub does round out the sound of the stock system nicely especially considering how poor the stereo was without it! I read in one forum that a guy didn't even notice the difference in his friend's car... Well, that guy probably hadn't set the gain and crossover frequency correctly. I'll repeat it, the sub DOES make a noticeable difference.
I wouldn't pay the list price, or anywhere near it, but considering I picked it up on eBay for under $100, it was well worth it - ESPECIALLY considering how easy the install was. Sure you might get more bang for your buck with aftermarket, but it's going to be a LOT more buck and a lot more effort to get it. $100 and an hour and a half for better sound? Worth it!