I'm sure a lot of people have read the Anandtech articles on Solid State Drives by now, and maybe even Joel Spolsky's story (the combination of the two is what convinced me, after all). Just how big of an impact will an SSD have on your day to day development performance? I can't answer the general case, but I can at least share my personal experience.
I've been using an 80GB Intel x25-m SSD for the last 4 months, and everything about my workflow is faster. The main tools I currently use daily: Eclipse, Maven, and Subversion, on Windows XP 64-bit.
building our full app with Maven (javac, lots of file copies, jar creation, and other related actions - a lot more than just compile):
500GB x 2 (RAID 1): 10 min 45 secs
80 GB SSD (after filling it up): 3 min 25 secs
full build w/o clean:
500GB x 2 (RAID 1): 8 min 32 secs
80 GB SSD (after filling it up): 1 min 42 secs
We're talking 3-4x faster, after the drive is full and at the reduced performance level it will remain at. If TRIM were present, this would be an even wider lead. Initially a full clean build took just under 2 minutes, as compared to 3 min 25 secs now.
Aside from rebooting to ensure nothing was cached and disabling antivirus scan, I took no special effort to make this a 'real' benchmark. It is only what I've observed, but is in no way scientific.
For Subversion I do not have any way to measure time, suffice to say - if you are familiar with opening a commit window in TortoiseSVN only to wait minutes while it calculates differences followed by finally listing your modified files, that is mostly a thing of a past with an SSD. 10-20 seconds is the usual max wait. For reference, this is on a 3GB workspace with 30,000 files. Note that the .svn directories double amt of files and size due to storing a 'pristine copy' of each file. The workspace, if exported, has 15,000 files.
Similarly with Eclipse, if refresh/clean/autocompile seemed to take a really long time - it is also a night and day difference on an SSD. Rarely am I waiting for Eclipse to finish now.
Having an SSD eliminates a lot of time I'd otherwise have to waste waiting for something to finish. I think they are a worthy investment, and they are getting cheaper every month. I bought mine only 4 months ago, and the price has dropped from $400 to $229 for an 80GB Intel SSD. There are also a lot of competitive drives in similar price ranges now.
Here are a couple of things to check when looking into an SSD:
Make sure your motherboard's SATA controller is modern enough and supports AHCI mode. The original, older machine I tried it in (it was a 3 1/2 to 4 yrs old Athlon X2) could not see the real benefits of the SSD. I think any Core 2 Duo/Quad or more recent motherboard will be fine.
Buy a drive with a firmware that supports TRIM, or from a vendor that has released a manual TRIM utility. I would not trust any vendor that promises to deliver TRIM firmware or a utility after-the-fact. I had the impression TRIM was coming with my drive, and it looks like I'm out of luck with my first gen Intel drive. Who knows what the timeline is for TRIM on the second gen Intel drives. I believe all recent drives w/ an Indillix controller are fine, and I believe OCZ is even working on a background-TRIM firmware that works in XP. (It will be some time before my work environment is Windows 7...). Also, if you are a non-windows user, pay special attention to the TRIM firmwares. I believe a lot of them may only support TRIM on NTFS.