I have never seen anything on an SSD getting slower when filled up, that happens on HDDs.
http://www.anandtech.com/showdoc.aspx?i=3531&cp=5
If you take the time to read all of it you will understand the difference between SLC (Single Cell) and MLC (Multi Cell) SSD's and how the cheaper MLC, the consumer level drives people are buying will simply degrade as they become full and wear out, not only slowing, with not so great write speeds to start but their read/write life span (MLC's) is 10x shorter than that of the SLC drives such as the Intel X25-E.
The article points out the differences most people buying SSD's fail to realize or even research and the fact that most companies use cheap controllers that will never deliver the speeds they advertise. If people are impressed with the fact their computer boots up 30% faster and that's why they buy em, go for it, but long term, which no one has really seen yet, but the way the drives store data won't change and over the next few years the degrading and failure rates will drop, opening people's eye's to the fact they spent $500 to $800 bucks on a SSD that is now slower than a traditional HDD that was a fraction of the cost.
I've read dozens of tech articles just like the one I listed the link to before purchasing my MBP and avoided the SSD for the simple fact they need to mature in both the controller technology and the SLC models to become a consumer product worth buying.
Can I afford a $800 SSD? sure, but I usually don't waste my money on something I clearly see as hype to get the tech junkies excited. My comment was to save someone that hasn't researched the topic some cash.
Bottom line is if people sit at their computer and start an app and close it 100x a day over and over.. and use a stopwatch to see how fast it open, by all means buy a SSD. If you like the rest of the world and open a application and use it or leave it open all day, the "cool" factor of saving 3 or 5 seconds to open an application quickly goes away as you realize your SSD is now 80% full because you could only afford the 128gig SSD and spent $500 bucks to do so. Using a little common sense to answer someone on the forums here that clearly is debating over $300 difference in MBP's by suggesting he spend $750 more as a "better" use of his money is insane and uneducated advice.