In my own honest opinion, Jailbreaking is really a personal choice, there are no dangers in doing it. I did some really dumb things with my 3g and 'crashed it and had to restore with custom ipsw ( due to 06.15 baseband on 4.2.1) but always restored it. Mainly you can unlock it but speed either way no noticable difference, and you can customise your device a lot more Jailbroken.
I used to just fool around with the root files and delete unused stock apps langauge packs which will speed the device up. I JB'ed 3g 3gs 4 4s all ok.
Alan