Installing cruise control is nearly unheard of. So many things to replace! I have cruise but I never use it... I can't imagine it would be such a priority. I suppose it's nice on the highway...
If you do a TON of highway driving, I suppose it would be a big help, but I'd recommend simply selling your car and buying something with a factory cruise control, instead of installing one. Honestly I don't even think it's possible.
Edit: http://www.honestjohn.co.uk/forum/post/index.htm?t=59737
Apparently it's possible, but not likely worth all the fuss.