Mbp 256 Ssd
My specs are below. I don't have know about my SSD to as far as real world numbers after degradation, etc... but I can tell you that the 256 SSD that came installed in my MBP is screeeeaming fast: from the push of the power button to a completed boot in... wait for it... 20 seconds. I've timed it on multiple occasions now... but, that is a characteristic of ALL SSDs (excellent speed when new). The question is how it will perform after the whole drive has been written to, when the firmware and controller installed in that particular drive have to delegate RE-writing those blocks of memory. The drives that Mac uses inside it's MBPs have historically not gotten great marks for this, but on the whole are still faster than even a 7200 rpm HDD after degradation.
As far as cost vs. time to install the drive (+shipping and +wait time if you're going to order your OEM SSD online), the math is much closer than it seems to be for the 128 and 256 gig drives installed by Apple. I think my cost comparison had Mac charging me about $50 (USD) more for their drive than a smaller one (200 gig) that I could have ordered from OCZ, which is the only other SSD manufacturer that I would have bought one from. And I didn't have to install it myself, an action which doesn't void your warranty but DOES expose the inner workings of your new expensive laptop to slip-ups / damage, etc. Additionally, I didn't have to wait longer to use my computer (waiting for a new drive to arrive in mail / reinstalling OSX, etc.).
Anyway, the moral of the story is that I am happy with my decision to take the plunge on the Mac 256 SSD. I'm trying to be a little judicious about what I save to my internal drive, i.e. using my external a little more often, but so far it's been all good.
Hope this helps.