In most cases its not worth it. Just my honest opinion, its a waste of money considering the costs, and time that goes into it. Especially if its more than what the car is worth. I guess there can be exceptions. If you find a used tranny somewhere and were able to swap it out.
I assume you have an auto? I do too, believe me not a day goes by I'm kicking myself for not getting a manual, I was but at the time I didn't have time to learn it as I was going to school and working. Plus my dad wanted me to get a new car ASAP cause he was tired of being driven around, and of course the dealer didn't have any manuals on the lot so I would have to order the car which in turn takes time. But the next time I'm in the market, I'm getting a manual!