What do you want the computer to do?
I have used cycle computers on my mountain bike. If I am planning a ride with my club, time, miles and climb become important - especially if I want to keep a good pace and not have the riders getting board.
In addition to these fetures, I have been looking at computers with heart rate monitors. Now a roadie will tell you that, after a while your heart rate becomes predictable or whatnot. However, there are some killer hills off road and keeping an eye on your heart rate is a good indicator of fitness.
However, I now tend to use my mobile phone. It has a program called sports tracker, which is linked to the phones gps system. This allows me to map my speed, distance climbed, fastest and slowest bits, as well at putting my route onto a map (with pickies).
To answer your question, it is worth doing, depending upon what info you need.