The downside is that the model of MBP that I own has a SATA interface that is limited to 1.5Gbps and the research that I did showed that the best SSD's are faster than that, so I'd be wasting some of the power of the SSD because the interface is slow.
The other problem is that installing a replacement hard disk in this machine is a nightmare. I've installed thousands of hard disks into computers and this was the hardest and most scary - I had to bend circuit boards and components to get a new 320GB/7200RPM disk in to replace the old 120GB/5400RPM, hoping that nothing snapped. Apple must have tried hard to design a machine with a disk that hard to upgrade.
Given those two issues I think I'd probably buy an entirely new machine and give the old machine to someone else in the family.
Second, which MBP do you have? That definitely doesn't sound right - I have a non-unibody 17" and it only takes about 5 minutes to take the top off; it's mainly just 12-18 screws, and the 15" looks identical to me (unless they changed how it's put together later on):
http://www.ifixit.com/Guide/Repair/Installing-MacBook-Pro-15...
Seriously, if you can at all stomach opening your laptop again, a 120GB-240GB SSD will make it seem like a brand-new machine. For the first week or so, you'll start ten applications at once just to watch how they all come up with no bounces on the dock. :-)
I have a non-unibody MBP from the 2007 era that I now have almost zero interest in upgrading; if it weren't for the fact mine is showing wear-and-tear and other improved hardware features (unibody construction, battery, display, improved headset port, audio-over-Displayport), it would be zero interest, since my 17" is now more than fast enough for everything I do. :)
I've heard Apple's SSD drives are expensive and there are better options, like http://eshop.macsales.com/shop/internal_storage/Mercury_Extr...
Depending on which MBP you have, you may even be able to fit a 12.5mm SATA drive, which means you could potentially squeeze in a 1TB drive.
Full details and benchmarks written up on my blog: http://blog.wpeterson.org/2010/12/05/macbook-pro-ssd-upgrade...
Overall about 5x improvement.