For a slightly-early birthday present to myself, as part of a post-Thanksgiving sale I bought myself an OWC Data Doubler w/ 240GB SSD. After making three full backups of my laptop, I installed it, and have been enjoying it quite a bit. This kit installs the SSD as a second drive, replacing the optical, allowing you to use it in addition to the standard drive, which in my case is a 500GB Seagate hybrid. I’ve set up the SSD as the boot drive, with the 500GB as /Users/Shared, and moved my iTunes, iPhoto, and Aperture libraries there, as well as my big VMware images and an 80GB Windows partition.

[side note: the Seagate hybrid drives do provide a small-but-visible improvement in boot/launch performance, but the bulk of your data doesn’t gain any benefit from it, and software updates erase the speed boost until the drive adjusts to the new usage pattern. Dual-boot doesn’t help, either. An easy upgrade, but not a big one, IMHO.]

Good:

• Straightforward installation. There was only one finicky bit where OWC's detailed instructions didn't quite match reality, which just required a little gentle fiddling to get the cable out of the way.
• Boots faster. Much.
• All applications launch faster, especially the ones that do annoying things like maintain their own font caches. Resource-intensive apps (pronounced "Photoshop") also get a nice speed boost for many operations, especially when I'm working with 24 megapixel raw images.
• Apple's gratuitous uncached preview icons render acceptably fast now. Honestly, I got so sick of delays caused by scanning large files to generate custom icons that I turned it off a long time ago (except the magic Dock view of the Downloads folder, which you can't disable it for).
• SuperDuper incremental backups are ~4x faster. 10-15% of this comes from not having to scan the 160GB of stuff that's now on a separate drive, but most of it is due to not seeking around on the disk to see what's changed. I've actually switched my backups from a fast FireWire800 enclosure to a portable USB2 drive, and I still save a lot of time.
• A little better battery life, a little less heat. The hard drive stays spun down most of the time unless I have iTunes running.
• External USB DVD drives work fine for ripping, burning, and OS installation.

Bad:

• Apple's DVD Player app refuses to launch at all unless an internal DVD drive is present; external drives aren't acceptable (unless there's also an internal, in which case you can cheerfully use both, and even have them set to different regions). VLC is a poor substitute. You can get it to work with external drives... by editing the binary. Seriously, you replace all instances of "Internal" with "External" in this file:
  /System/Library/Frameworks/DVDPlayback.framework/Versions/A/DVDPlayback
• Time Machine backups don't pick up the second drive. Neither does SuperDuper, so I added a one-line script to my SuperDuper config that does:
  rsync -v -‍-delete -‍-exclude .Spotlight-V100
      -‍-exclude .Trashes -‍-exclude .fseventsd
      /Users/Shared/ /Volumes/Back2/
• Snow Leopard seems to have lost the ability to reliably specify the mount location of a second drive. In previous releases, you could put the UUID or label into /etc/fstab and it worked. Now that file only accepts device names, which are generated dynamically on boot. This works if only two drives are present at boot time, since the boot drive will always get disk0, but having an external drive connected could result in a surprise.
• (obvious) Can't watch actual DVDs without carrying around an external drive.

So, file this little experiment under “expensive but worth it”. I do watch DVDs on my laptop, but only at home or in hotels, so the external drive isn’t a daily-carry accessory. The SSD has a Sandforce chipset and 7% over-provisioning, and is less than half full, so there’s no sign of performance degradation, and I don’t expect any. Aperture supports multiple libraries, so I can edit fresh material on the SSD, then move it to the hard drive when I’m done with it. Honestly, unless Apple releases MacBook Pro models that wil take more than 8GB of RAM, I really see no need to buy a new one for quite a while.