I had a mechanical thumb on my BH92 hoe and converted it to hydraulic. I found it only slightly more useful for moving rock and regretted spending the money.
This of course depends on what type of job you're doing and I'm sure others will disagree. In my case, I was piling +-200# rocks to make a wall. Picking up the rock is one thing but then you have to transport and get it into place. After wasting a couple of days climbing on and off the tractor & backhoe, I decided to do what the pro's do and rented a mini ex. The reach and lift ability are far superior to a CUT backhoe and the whole operation can be done from the same seat. I did the whole wall in less than a day with the mini ex. It would have taken a week with the hoe.
Unless you have other jobs that require a hyd. thumb, I'd spend the money on a rental instead.