Damn straight it's worth it.
As a result of recently getting a 2011 17" MBP, with the Apple 128GB SSD, I couldn't resist upgrading my 2 2007 iMacs.
The iMacs were getting pretty slow, that's one reason why I bought the MBP, I needed some portability, too.
I just finished upgrading the 24" iMac yesterday, which was my primary machine. I installed a 3.5" 120GB Vertex 2. I also put a 750GB Scorpio in the Optical spot. Good grief, the darn thing is a 4 year old new machine.
Yes, the processor isn't new, but for just $300 in parts, it's awesome again.
Prior to the big change I put the SL install DVD on a thumb drive. Slick stuff. Did the same with the new MBP install DVD.
Going to put a 750Gb in the MBP's Optical spot as well.
My daughter gets the 24" as we both will be programming for iOS, and working in Unity3D as well.