Is it worth it to upgrade a mid-2007 MacBook (MB061LL/A) to use an SSD?
Yes,
... because there are other advantages besides the faster access times.

SSDs work inaudible
SSDs are shock-resitant
SSDs use less energy but the total impact is very little...
SSDs max out the SATA 1 throughput, which your old HDD will probably not.

Also, you might get a SSD and then use it in your newer Mac which you will buy in a few years. Make sure to get a SSD with a SATA 3 interface - which is backward compatible to SATA 1.
What about TRIM? The current SSDs have very efficient garbage collection which make the need for TRIM dispensable. But as such performances largely differ depending on the hardware manufacturer, make sure to study performance reviews of the SSD which you'll consider.
I've written some more in this post.